I'm caught in a very awkward triangle. Over a year ago I introduced a guy and a girl, both close friends of mine, and they hit it off. They went from zero to "I love you" in three months flat and I thought things were perfect. The problem is that my guy has a bit of a problem. Years ago, he fell into the "being an 18 year old guy with a 16 year old girl" trap and to make a long story short ended up plea-bargaining and landing on the sex offender registry for 5 years. This initially did not stop things as he is completely open and honest about it. However this limits the time they can see each other, which is where we now run into problems. He is 100% trusting and loyal, but even though he is on his last few months of registry, she's developed a bit of a wandering eye. Or to put it simply, she cheats on him constantly, even trying to drunkenly seduce me. She tells me it's "just a phase" of her dealing with being away from him so much. I've told her to just dump him if she's going to sleep her away around, but she claims she still loves him. He is completely oblivious.
At what point do I just come out and tell him that the apple of is rotten?
Languishing Over Letting Woman Hurt Admirable Truster
Just so we're clear: your friend's problem isn't that he's on a sex-offender registry—which he shouldn't be (please read this for background on the abuse/uselessness of sex-offender registries)—but that his girlfriend is a CPOS.
You owe it to your guy friend to tell him what your girl friend—his girlfriend—has been getting up to in his enforced absence, LOLWHAT, even if it costs you the friendship of one very messy drunk with a fondness for bullshit rationalizations. Your guy friend has suffered enough over the last five years, I'm thinking, and the last thing he needs is a girlfriend who cheats on him constantly and hits on their mutual friends when she's hammered—unless, of course, he likes that sort of thing.
And if he does like that sort of thing—and lots of men do—that's likely to come out during the conversations your guy friend has with your girl friend once you do the right thing and let him know what's been going on.