What are some arguments in favor of cheating?

I'm talking relationship cheating, like, screwing somebody who is not your girl/boyfriend. I'm not condoning it and I certainly did not do it myself. I'm asking because a friend of mine cheated on her boyfriend, who is gigantic, horrible cockflop, and she's feeling bad about it. She won't understand that he's a piece of shit so I figure I should just try to make her feel better about cheating.

So far my arguments have not been very good. I want her to feel better about this. It was a good decision for her, perhaps executed poorly, but ultimately, it's a good decision.

And no, she did not cheat with me. Some other dude I don't know very well. He can't be worse than her boyfriend though.