I have an NPC who was supposed to be a twist villain - helped the party by carrying loot and brewing potions, but tried to steal their magical equipment once they had enough to make it worthwhile to her - that has become something like a friendly enemy.

I figured that they would just kill her once they beat her. However, the players won and then actually listened to her explanation.

To my complete shock, my players gave her a share of loot and the Paladin - a Devotion Oath who lives by the phrase “lawful good doesn’t mean lawful nice” - added the demand “You will attend a service at the nearest Temple of Bahamut and reflect upon your actions, and the light shall show you that… (blah blah justice blah blah).”

So the NPC magically disguised herself as that same Paladin and diligently participated in the religious service. The moment the congregation was dismissed, the NPC sprayed graffiti, comitted vandalism, and otherwise trashed the place without hurting anyone. “Nobody tells her what to do” was my thinking at the time. I wonder if that was too much, but whatever. It’s done. 

When the party returned, the Paladin was shocked when he was arrested and nearly excommunicated. Of course he was eventually cleared, but the interrogation was an interesting twist on a normally combat-heavy campaign. 

That same Paladin is about to get married to a different NPC - a childhood friend he finally rescued after a long quest. I have absolutely no idea why, but he’s requested that the NPC who trashed the temple can be his best man if she fixes what she destroyed in the temple. And that’s where we left off. 

I never intended this chaotic twist NPC to live this long, and I have no idea what to do next. Several ideas which floating around are:

(1) Refuse the offer - “I do what I want and I don’t care about your stupid wedding.”

(2) Twist the request - agree but use magic to anchor all the broken parts of the temple so that they are permanently in a broken state (because the word “fix” means “to fasten securely in place”) and then laugh in their faces and demand to be the Paladin’s best man anyway 

(3) Start down a better path - cheesy, but maybe the NPC can slowly become better by fixing what she broke and participating in the wedding sincerely (this has the added benefit of making the players guess “what the heck is she up to this time?” When she’s actually not up to anything this time)

(4) Trick the request - fulfill the request to fix the temple, but do so in a way that causes other problems down the road: that new golden reliquary? Stolen from the next town over; the stained glass that glistens with iridescent colors? The spirits of several pixies are trapped within the glass and an emissary from the Feywild is coming to free them soon; All the new holy gilded books? There’s a penis drawing in every one of them on a random page; the new door? Haunted. 

I realize that’s a lot, and my table is never going to be like anyone else’s table. This is my first time trying out an NPC that’s not black-and-white honorable good or dyed-in-the-wool evil.

Basically: help me continue to be entertaining and consistent without crossing the line and being a d!ck. 

I would probably start by asking the player what they're looking for, and then decide whether you're willing to give them what they want. Also, think about what the NPC actually is; is she a relatively normal person who just happens to be dishonest, or something like a chaotic trickster spirit from the feywild? In the first case my default would probably be some variant on #1 (possibly more like "You have got to be kidding" than insulting), in the latter case #2 and #4 seem more likely.

I would personally have them help clean up but after the ceremony they rob the church. It's a bit of a best of both worlds.

I kind of love it when players get attached to some NPC I wasn’t even going to bother giving a name to. 

I might actually take your option 3. The players really seem invested in making this particular NPC turn around, and they’ve been putting in the work. Maybe give them the win. You could start slow, like the NPC turns gown the best man role, since I’m not getting that vibe from them. But they do make a quick appearance. Just long enough that the party knows they’re there. Probably the PC’s paranoia sets in, and turns things interesting, but the NPC is being real this time.

And then even if they’re not fully a good person, they’ve grown to like, or at least respect the PCs. So they might do bad things but not to the party. And if there are more interactions, they could turn more friendly. Maybe they become an informant if they think someone else is going to hurt the party. Maybe the NPC becomes a weird guardian angel for the paladin’s spouse while pally is off adventuring. But even while doing that, they still rob and con other people.  They just consider the party to be their friends.

I'd try to look at it from the perspective of the NPC - not a cartoonish caricature but try to imagine the character of the NPC that you created in the first place. What motivates them, what is their background, what are they looking for in life?

The NPC in this case found a fairly gullible party that allowed them to tag along in exchange for some services - carrying loot and brewing potions. Presumably they were being paid. Their intent was to wait until it was worthwhile then run off with the loot. Why? It sounds like they wanted money to live however they want OR do they have something that they require a lot of money for? Maybe they want to resurrect their mother who died in childbirth who they never got to meet or maybe they give some of the money to the orphanage where they grew up or to the poor farmer parents she ran away from because she didn't want to marry the local country bumpkin or maybe she just wants to be rich?

Once you know WHY she wanted to steal stuff from the party and a bit of her backstory, you will have a better idea of how the NPC will react to the unexpected mercy and kindness from the party after being caught stealing.

From your description, the NPC is not evil. They aren't motivated by a drive for power, financial domination, or a desire to inflict suffering on others by taking things from them. The NPC does sound chaotic, they don't respect other's property and very strongly believe that no one should tell them what to do. On the other hand, disguising themselves as the paladin and then trashing the temple is a bit childish. A simple Zone of Truth will reveal the paladin's innocence (and a Devotion paladin always has that one prepared so they know it exists), so in the long run the act of "rebellion" really doesn't do anything except irritate people and the NPC doesn't have a reasonable expectation of remaining anonymous. This implies that the NPC is impetuous and really doesn't think too much about consequences for their actions.

So, the next step is to ask yourself how this NPC will respond to the exceptional level of kindness and generosity demonstrated by the paladin after their betrayal and acts of rebellion ... keeping in mind that the NPC has likely been with the party for a substantial length of time (weeks, months, years?) and likely knows all of the characters well from their day to day interactions. Does the NPC consider the party members "friends"? Has the NPC gotten close to some of the characters in the party?

Clearly, the paladin's reaction (and the other party members?) to the actions of the NPC are more of how they might choose to treat an errant sibling rather than a complete stranger. Does that feeling of friendship and possibly family that the paladin appears to be clearly extending to the NPC mean anything to them? How does the NPC react to a group of people that seem to want them to stay even after they have tried to rob them AND embarrass the paladin? If the NPC isn't exceptionally selfish or evil they might see becoming a party sidekick as a good way to earn coin without having to steal from possibly new found friends. Besides they can always steal from someone else when the opportunity comes up, if they just can't avoid the urge to take things :)

Anyway, you can justify pretty much any of the outcomes you described ... however, depending on the NPC character and motivations ... they might decide that their most profitable course of action would be to work with the party, earn treasure on their adventures with little risk, pay lip service to the paladin's requests and perhaps see what it is like to have some friends.

Thank you all for your input thus far. It’s all certainly been useful. I hadn’t intended the four options I mentioned to be a choice as much as examples. 

If anyone can think of others, I am still open to ideas. 
I also felt (afterward) that trashing the temple came across as more childish and tantrum-like than I wanted this NPC to be, but it’s done and I want to keep things consistent.

Considering that the NPC’s actions have demonstrated (as several of your mentioned) a lack of long-term planning, I am still leaning toward doing something ill-advised and unexpected.

But now the PCs are expecting the unexpected so maybe I should do the expected, which is to be unexpected. 

I think I confused myself. 

Regardless, I appreciate everyone’s input and I am still open to further suggestions and new ideas. 

I think there are times when you as a DM should step back from "what makes sense for this NPC" and instead focus on "what story are my players trying to tell". This Paladin clearly wants to attempt to redeem this person who has wronged the party. It could be interesting to have them be vexed at every turn by this NPC who only twists their requests and brings evil, but if the player is reaching for something like this, I tend to let it play out closer to how they want. 

You can still have them trash the temple or not attend the wedding, but give the paladin enough to let them know that the woman isn't beyond redemption. If it's something they want to work towards, that should be rewarded, rather than shot down. 

Edit to add this final bit: remember that in the end DND is a game we play with friends where we tell a story together. Helping your players tell the story they want to is a really good feeling.