Using a friends account...

Ok so my account has been banned and my friend is willing to let me create an ad account from him and use it. What I don't understand of people here that have done this is if you are supposed to log in as him forever (which would basically mean he cannot use the account for his personal stuff anymore) or if you should just add yourself as an administrator of the business account. If you should add yourself as admin, then do you need to warm up the admin account too? if yes it would defeat the whole purpose i think. thx