Should I tell?

[phisigduchesscv]

As much as you want to tell the guy don't!!!! It's the messenger that will be shot not the girl who is making out with other guys.
The guy won't believe you and she will be so mad at you for telling that you will have lost her as a friend for good (said from experience). She'll end up saying yes to the guy just to prove a point.

The only thing I suggest is that you talk to her about what she's doing and ask her is it really fair to her child - is that the type of relationship she wants as an example. Play up that her child is going to think that type of relationship is how it should be and will never learn what it is and how to be in a loving relationship.

Also, the other thing is he may already know and wants to be blind to it. If she's making out in bars then some friends of his have been around those same bars and may have told him. Whatever you do, do NOT say anything to him.