How can I tell my mom that I think dating an engaged man is wrong?

I am a 17 year old, and my widowed mother recently told me she's dating a man. The problem is, the man has an out-of-state fianc�e. She's deluded herself (with the aide of this man) into believing those two are virtually broken up because they quote "hardly talk anymore", but I find the whole situation disgusting. It might not be so terrible if she wasn't being such a hypocrite (she's a "strong christian" and never fails to point out when someone else is "sinning"). How can I tell my mother that what she's doing is wrong? Should I tell her at all? Btw, I'm not a religious nut; I'm actually agnostic, but I have strong morals when it comes to hurting anyone, emotionally or physically.