I think if he gave you his proof of purchase like receipts and such then you wouldn't have to go through him as the middle man. I could be wrong but I think it would work.And the whole lending of his account is weird to me. I think he should hand it over to you permanently imo
It sounds like a headache to me. im assuming you bought games on his account? If you wanted to say the hell with the warranty then you could offer him some more money for the games he bought, not at full price . But you are in a pickle! I hope you can both work this out .. good luck!