Never mag dump someone else's rifle without permission. It's rude and liable to make it so other firearms are not lent to you to try in the future.
I suspect most of us know that but hand you buddy that has only shot a AR playing Call of Duty your gun and you never know. Sure you ask him if he has shot one before and knows what he is doing and he says yes. Your first clue should be when he ask where the R key is for reload. Sorry guys I have played all those games even in my late 60s. Don