My friend is a womanizer!?

My friend, we'll call him Jeff, is more of a womanizer than anyone you'll meet. He talks to one girl and flirts with her for a while, and then they leave the party, he finds another girl to talk to. He had a gf a few months ago and would talk about every time they hooked up, right after it happened. It made me and my other friends mad, but we didnt say anything. He's currently trying to get with one of my friends who I actually really like because she's a really cool person, we'll call her Stacy. He talks to Stacy like he's interested in her, but just the other day he hooked up with another girl who's just a "friend". How do I let Stacy know that Jeff is not the best bf without befriending Jeff. How can I tell Jeff that people think he's just trying to hook up with anything that moves without seeming like I'm invading his privacy? What should I do?