Um, a $500 rifle will break with green gas if its supposed to be 134a only. There are reasons for the different kinds of gas. The m1garand isnt supposed to take green gas unmodified. Thats why all sites tell you that if you use the wrong gas, you do so at your own risk.
With that said. Where ever he bought it from probably wont take it back, since almost every site that carries the rifle states its a 134a gun. And he didnt listen to their warnings.
Its like buying a CD player and in that players manual it says to only use a 6vlt wall adapter for constant power, and then using a 9vlt one on it and blowing it, then wanting a refund.