she is coming to you 2-3 months after the fact?
you owe her NOTHING.
Rabbit showed for you successfully.
And bad teeth are checked for at shows.
If she had bad teeth now it won't be due to lack of hay, it will be due to rabbit pulling on the wire of the cage or something.
It is NOT your issue.
DO NOT refund money.
it's one thing if she brought the rabbit home and then said HOLD ON.. I just saw something we missed.
BUT as you stated. You checked the rabbit over before sale. the buyer checked the rabbit over before sale.
Rabbit is sold AS IS. no guarantees on future showing ability, no guarantees on the ability to reproduce or anything.
Just tell her how sorry you are about the situation but you are NOT in the financial position to pay for the rabbit.
You can't fix every problem.
if the rabbit had a problem earlier she should have told you sooner.. not 2-3 months after the fact.
If she didn't know the rabbit had a tooth problem then she's not observing her rabbits as closely as she should. you owe her nothing.