Let me get this straight. You are no longer friends with D the girl being cheated on but T the girl doing the cheating with the boyfriend is who you visit frequently.
I'd advise you to look at peoples character a little more closely. People often say things like I don't want to judge or I'm not perfect myself.
You don't have to be perfect and it's not judgemental to have some discernment in your life. Anyone whom you consider a friend is not going to be the type to put you in the position of knowing something this awful especially about someone whom you were once friends with.
T doesn't give a damn about your morals or your values as a human being let alone friend. If she feels it's okay for you to know all this dirt about her scamming. If she is sleeping with somebody elses boyfriend please trust she doesn't draw the line there. This girl doesn't know the meaning of boundaries much less care to respect them.
you should tell the girlfriend because she doesn't deserve to be made a fool of and she deserves to have a choice in this matter. So far the ho bag of a boyfriend and the ho bag he is sleeping with are the only two in this "relationship" who get to make choices based on what's really going on. The girlfriend deserves to get to make an informed decision as to:
Whether she wants to continue sleeping with a man who is sleeping around.
Whether she wants to continue in a relationship that's not monogamous.
Whether she wants to be friends with someone of such little character.
Right now she is being deprived of that. Show YOUR character. lying by omission is wrong. She has the right to know even more than you do. After all when the pieces fall you can sit back and watch it. She has to LIVE it. Give this girl a chance in hell to make some decisions about this situation that can help her.
And raise your standards for whom you call friends. If that sounds like your mother or somebody so what. Mothers are usually right.