Guy I'm seeing is "best friends" with a girl who is known to sleep around?

[=)]

Well, I've been dating this guy for the past month. Everything is going by great; we're pretty much a couple, though not "official". I can tell he likes me, and I really like him.
So there's this girl who he talks to. First, she followed me on Twitter.. we had mutual followers so I didn't mind. Then, she added me on Facebook (which was before the guy even did). One week later, I finally met her, and apparently she considers him her best friend.
Then a few weeks back, my friend comes into my room, warning me about her. She said she realizes that on Twitter she mentions him a lot, and on instagrm she posted this picture with him, saying "My bestfriend <3"... again, I didn't care really since I trust him.
SAME NIGHT. He had a movie thing in his room, and I had went. Once the movie had begun, the girl was talking. The guy playfully told her to "shut up". So the girl, with her booty shorts and all (lol), gets off of the bed, and tackles him on the bed, legs sprawled around him and everything. He was laughing and playing along. I was really annoyed, but I didn't say anything, though I did end up leaving early.
Ever since, he still has those movie night things, but he always invites me last minute (it's usually a group of his friends), though for some reason she's always there (though, according to my friend, she invites herself places a lot). I haven't gone since the last time.
Then today, I was talking to my friends about it, and it comes to find out, this girl is known to sleep around with guys randomly and even homewreck relationships. I go to a big university, but somehow every freshmen knows everyone lol, so she told me a couple of stories, but at the same time she told me not to worry about her because "she's not that pretty anyways".

I don't know, now I feel some type of way about this. Some people say I should talk to him about it - but about what? We're not boyfriend and girlfriend. It's just bothering me though... Advice?