I know you are being cool, which is great. I think it's the right thing, if you can help him out.
But get something in writing from him saying he received in perfect condition and the damage was caused by his own fault.
This attempt to repair is a totally seperate transaction and the buyer understands that.
(maybe the email chain is enough, but asking him to sign something to send along with the frame is not a bad idea)
If it gets ugly, it could be seen as you agreeing to fix the damage as a condition of the sale if it is not written down.
If you can't warranty it, suggest he give
http://predatorcycling.com/ a call. They do carbon repairs here in socal.
(no affliation, but know they do the work)