If it comes down to that, and I hope it doesn't, there is also the concept of "Due Diligence" on his part...Did he take measures to assure the bike was in the condition he assumed it to be before leaving with it?
If he didn't then it's your word against his, he didn't prove it was good before he left with it..bought it hoping it was what he wanted...He just He bought it "As Is".......
I just did some web surfing and it looks like Georgia Private vehicle sales are not covered by any "implied" warranty like a dealer would be held to. Its "As Is" unless specific written warranty was made by seller at time of sale...........And of course you can believe everything you read on the internet
I like the offering to pay for parts idea, although before I did that the parts oughta be named specifically or it could open a whole other can of problems if some mechanic dives in and comes up with a bullshit list of parts involved....