Here are a few pointers where the dude can sue your friend if..
1.They have intention to create legal relations.
2.An agreement (Offer and Acceptance)
3.Consideration
4.They have capacity
5.They were not affected by reality of contract
I assume this scenario is classified under Commercial agreement.If they do not have a contract,the case is deem as a gentlemen's agreement.In other words agreement was binding in honour.So no contract equals court wont bother.
Another point I'd like to make about the Coke/Pepsi or Guitar/Piano scenario is that its classified under Common Mistake.
Both knew the agreement was about a guitar of particular specs and the transaction was about a guitar of that particular specs.So in conclusion no mistake hence no chance of refund/filing of law suit or whatever.
My advice to you for your friend is that the case here is never gonna make it in court.The judge is gonna have easy money on this case.
Im having Business Law test tomorrow.Thank god i can refresh my mind with this thread.