Rules on Victory?
 

Is there any rule prohibiting someone from aiding another person in the game winning level? For instance if I had my friend help me in battle, and he did not realize that I was about to win, would this be fair? Also is there a rule allowing him to back out, or is it final?

Re: Rules on Victory?
 

There is no rule against having a helper in a fight for the win (although some cards and abilities that can compel a helper exclude combats for the winning level). A helper cannot back out of a fight unless he has a card or ability that allows him to.

Of course, your helper is allowed to work against you if he can -- he can play monster enhancers or one-shot cards to try to make you (and him) lose.