My friend is being a womenizer and i want to get him to stop?

Don't just treat the symptom, cure the dissease by talking to his girl, make her see what a duche he is, get her away from him...you take the victim away from the bully, he's a bully no more.

My idea is don't bother.

I don't know what your relationship is to this man, So it is hard to give you any type of advice without this knowledge.

Do you live across the street from him, are you a co-worker, how ell do you know his wife if he is married. and what will you get out of exposing him?

We teach people how to treat us. In other words, you need to make it clear that this behaviour is unacceptable to you and you will not tolerate it.
You cannot change him, he has to do this himself, but you can change how you are treated. If he is not willing to change out of respect for your wishes, ask yourself if you really want to be with such a man. You do deserve better. You should believe that firmly and don't ever accept a second best relationship.
A relationship should make you happy. It is far better to be single than in an unhappy relationship.'
Give him the chance to make a change, explain clearly how this makes you feel and what you want from him. Then go from there. If he loves you and respects you, he will stop doing this. If not, truly consider moving on - the longer you stay with a man who is not showing you respect, dignity and love, the less time you have to spend with the man out there who will show you all these things. Don't waste time on a loser.

Talk to him, don't rat him out....that's not very friendly to me.

haha this made me laugh, umm heres my answer: introduce him to another person like hmmm, preferably someone who has a STD and then I am sure he will learn his lesson JK