Stealing Account

So i currently have an alt account on my hands that a '' guy i played with '' gave me, he said he is lending me the alt because he can get it back simply, So i was wondering, is there a way i could steal it without him getting it back?

Get the session ID via the minecraft logs. However, I think session ID's expire after a couple days. There is no real way you can hack it unless the owner gave you mail access or the account has no security questions. And even then, if the owner has the transaction ID, he can easily get it back too