Let's suppose a Player Character is a Demon or Devil or something, and has a Weakness to Holy/Divine Magic. Now let's suppose a Cleric/Priest(ess) attempts to heal them with magic. What would be the most likely result?

Clarification: It's a weakness to Holy/Divine Magic in general, not just Radiant Damage.

• A:The Healing Magic Harms them.
• B:The Healing Magic does Nothing; neither harmful nore helpful.
• C:The Healing Magic heals Double the amount it normally would.

I mean, this is well outside of 5e's paradigm, particularly since there is no mechanically defined "holy/divine magic". The term probably gets tossed around a few places, but to the best of my knowledge there's no creatures or other rules/effects that check for or work differently based on whether or not a spell is "divine magic". RAW, the 2014 Cure Wounds spell only specifies "This spell has no effect on undead or constructs", and the 2024 has no conditionals, so there's no reason they should function differently for a Fiend. Now, this is by RAW; if you're homebrewing, you do you.

so, to make it make sense in a 5e way, i would define weakness to divine magic in the following ways:

• creatures with weakness to Divine Magic have Disadvantage on Saves against Divine Magic
• Divine Magic has Advantage on Attack rolls against creatures with weakness to Divine Magic
• creatures with weakness to Divine Magic take double damage from Divine Magic
• creatures with weakness to Divine Magic gain no healing from Divine Magic

Making healing also harm them is an overkill at that point. And making it heal double makes no sense if every other application of that weakness is detrimental to the creature.

I think the best answer is not to think of this as a mechanical issue but as a narrative issue. RAW makes no distinction here, but we need to remember that the rules are merely a framework to help us tell a good story. This is a wonderful opportunity for the player and the DM to establish some "vague & evocative" (Dael™) guidelines for this. 

Is the character a full-fledged fiend, or just a tiefling? I think the amount of fiendish blood would determine the intensity of the response. Let's say for the sake of argument that the player is using the tiefling page as the basis for character stats, with a DM-approved understanding that the fiendish side of the bloodline heavily outweighs the human/mortal/etc side of the bloodline. 

I think a fun idea would be that Holy/Divine Magic (however you choose to define that) has the expected result - the healing spell heals the character, the Bless spell buffs them, etc. BUT... when the spell takes effect, there is a noticeable side effect that suppresses the fiendish blood for a moment.

The tiefling character gets hit hard and begins to stagger. The party cleric reaches out a hand, touches the tiefling on the arm and casts Cure Wounds. The tiefling regains hit points, but their arm changes in form and appearance from their usual fiendish look to a more human look, their sharp fingernails become dull, and they exude a slightly smoky smell. But after a round or two everything returns to normal.

Or if you want to go more intense, maybe every time the tiefling receives healing magic it burns, so the character regains those hit points MINUS one point of fire damage as the Divine Magic literally burns itself into their flesh. 

Or maybe whenever the tiefling character receives magical healing they regain the HP but it hurts so bad that they also have to make a saving throw of some kind (probably Con) or else have disadvantage on their next d20 roll due to the pain, or maybe just a general queasiness. 

This could be a really fun way of providing character distinction and role-playing opportunities without seriously hindering character survival. Have fun with it. See what happens.

The short answer is - There shouldn't be any change in interaction. The spell does what it says it does.

The long answer is more complicated.
In earlier editions like 3.0/3.5 Healing spells could be used offensively against some forms of undead or other creatures with strong ties to the Negative Energy Plane, wherein the damage type was Positive. Similarly, 'Inflict' spells that did, what was at the time, Negative Energy damage provided healing to most of those creatures.
You had various other energy/damage types added in further supplements, like Vile damage, Divine Damage, or, because of how certain damage reductions and immunities worked, Lawful/Chaotic and Good/Evil *aligned* damage.

The important thing to consider is that healing is not, by the definitions of the system overall, 'good', nor historically have divine sources of healing had any additional major functional effects on fiends or other extraplanar creatures, evil or otherwise!

In the current edition(s), the various extraplanar energies were streamlined - Radiant and Necrotic.
They still carry the connotations and implied links to the Positive and Negative Energy Planes, but most undead, some fiends and more broadly, lower planar creatures will do Necrotic damage, and Celestials and... really, anything else Outer Planar will do Radiant. There are instances where the inverse is true, but neither does anything additional to one another aside from being a reliable damage type to use against most planar creatures.

Fiends can be healed. There's really no standing argument for why they couldn't or shouldn't be able to benefit from healing spells. Mechanically, it really doesn't make any difference if the caster is the most devout paladin of Lathander or the most wretched cultist of Demogorgon. The spell does what it says.

From a player perspective, if someone at your table is playing a tiefling, cambion or - for whatever reason - an actual fiend (Ignoring the implications of balance or thematics), telling them "Hey, I'm houseruling that cleric/paladin/druid/ranger/Any-Number-Of-Subclasses can't heal you!" is a clear sign that they ought to find another group. If they want to roleplay a discomfort, that's entirely up to them, but it shouldn't have any functional impact on gameplay.

The only reference that I’m aware of is from 3rd edition where healing spells would damage undead creatures.  

There is no differing effect in 5E that i know of.

Some healing spells specify that they don't work on constructs or undead. But nothing is said on effects on healing magic on unholy things.

Older editions, as well as pathfinder do have rules that healing harms undead, but that is all not applicable to the current iteration unless you want to homebrew or table rule it in.