Is it okay to say "that's cheating!" instead of "no fair!" in this context?

Native speaker’s answer
Rebecca
Well, it's possible if it's against the rules! However, it seems like this is more of a "loophole" than "cheating." A "loophole" is a permittable or legal way to avoid or escape something or find another way to do it. And it's not fair since the other plans can't do this. If it was clearly against the rules, then yes, he could have said, "that's cheating!" Ex: I caught him trying to cheat during the card game. Ex: I found a loophole in the contract.