My player's Graviturgist Wizard wants to yoink people around a whole bunch.

My question is timing.

Essentially, she wants to hit something with Vortex Warp, place it "safely" near a cliff, fire, etc., and then shove it another 5 feet with Gravity Well. 

My confusion is on Gravity Well's wording of "whenever you cast" vs. GW triggering when the spell hits or its target fails a save.

Gravity Well: "You’ve learned how to manipulate gravity around a living being: whenever you cast a spell on a creature, you can move the target 5 feet to an unoccupied space of your choice if the target is willing to move, the spell hits it with an attack, or it fails a saving throw against the spell."

Vortex Warp: "You magically twist space around another creature you can see within range. The target must succeed on a Constitution saving throw (the target can choose to fail), or the target is teleported to an unoccupied space of your choice that you can see within range. The chosen space must be on a surface or in a liquid that can support the target without the target having to squeeze."

"Whenever you cast" strikes me as "resolving" first, so visually/thematically like pushing an ally out of danger and then healing them, dragging a pile of creatures into the epicenter of a fireball, nudging a creature 'into' a Vortex Warp, etc.

The "spell hits" and "fails a save" seems to indicate the triggering spell "resolves" first. So in the above examples, the ally is healed then moved, the fireball additionally yanks everyone around, and GW moves a creature 5 feet after being Vortex Warped 90 feet (and thus allowing the Graviturgical pinball machine my wizard wants to set up). The same question could be asked re: Telekinesis, I think.

TIA. Sorry if this has been hashed out; pointing to previous threads is great, too. I'm coming up empty on searches.

The way I read it is that the Gravity Well effect is "activated" by a spell being cast, but it is actually resolved once the save is failed. In other words, the spell being cast is a condition that needs to be met before you can use Gravity Well, but the target being willing, being hit, or failing the save is the actual trigger to resolve it.

You're right to wonder about how that applies when dealing with two different "move" type effects though, i.e- which one happens first when they both have the same trigger (the save being failed). There is an optional rule in Xanathar's Guide to Everything on Simultaneous Effects:

If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen.

So if the player is doing this during their own turn, they would decide the order so applying the push second would be valid, but if they cast the spell using the Ready action then it's technically the target's turn so they could decide the order instead, though it depends how you think of a reaction since the reacting character is the one that's actually acting at that moment.

Personally I'd allow it, but either be careful about offering up these opportunities to do it (can't throw someone off a cliff if there's no cliff, you're indoors or whatever). Exploiting environmental hazards is a lot of fun, so I wouldn't stifle it completely, just limit access so it doesn't just become a routine.