I've recently found myself in a predicament with a very good friend and colleague of mine. I felt as though I handled it the way I should have, but I keep feeling as though I could do more.

This friend of mine got engaged to a woman that he fell head over heels for. They moved in together within 3 months and became engaged within 9 months of meeting. They both work in the same field and have many mutual friends (myself included). Now heres where it gets sticky. I was friends with him first, friends with her thru other colleagues second.

I found out within 4 months of them meeting, that she had been flirting with other colleagues behind his back. Going out with other male colleagues behind his back. Sexting another colleague - you guessed it, behind his back. A mutual friend attempted to tell him all of this and his response was "go fuck yourself, she would never do that." Talk about rose colored glasses eh?

Well, shocker - he begins to realize all isn't well in the engagement and came to me to vent. During this vent, I told him that I knew she wasn't behaving appropriately. His response? "You bitch. You're supposed to be my friend. You're supposed to tell me these things!" To which I told him that if his head wasn't stuck up her vag, I would have.

So, my question is... if you know the significant other of a good friend of yours, is behaving inappropriately - when do you tell them? Or do you?