tl;dr - Friend cheating on other friend, wat do?

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Hey /adv/,

So I need an opinion on a moral dilemma my female friend Anna is facing. Our mutual friend of 7+ years, let's call him Mac, has been dating this girl Iris (another long-term mutual friend of both of ours) for about 5 years. Last year, they broke up temporarily because Mac essentially said he wanted to spend part of his college time sleeping with other women.

He ended up having sex with one of Iris's close friends, and it turned into a huge argument. She and Iris still don't speak, although Iris and Mac got back together shortly after. Still, even after they got back together, I'd be at parties with Mac and he'd try to have sex with other girls, who on more than one occasion refused when they found out one way or the other that he had a long-term girlfriend. Iris is super sweet and naive, she has no idea - they live 90 minutes away from each other now after Mac dropped out of college this spring.

But about 2 weeks ago, Mac had sex with a 42 year old he met at a bar (we're 23) and told me, Anna, and my girlfriend about it. We were all pretty drunk, and he tried to kiss Anna and admitted to having feelings about her (though he says he doesn't remember doing so). He hits on her constantly when Iris isn't around.

He and Iris were planning on moving in with each other within the next few months. The whole cheating thing really bothered Anna, and as a good friend of both, she decided to tell Mac that he had to either tell Iris about what he had done or she'd tell Iris herself. Mac blew up on her and started attacking her character, refused to accept any wrongdoing, forced Anna's hand and acted like he's the victim in this situation.

So /adv/, should she tell Iris, almost certainly end her friendship with Mac AND end their relationship? Or should she keep it quiet and let them move in together despite all the things Mac has done?