It is up to you.
Personally, I would message the buyer and remind them they got a good price but, if they are unhappy with the guitar, offer to have it back and refund them.
I would not offer anything towards the repair.
Give them a 'keep it or return it' choice.
As stated, outside of 30 days, buyers don't have any right to return an item so you could just ignore the buyer.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)