What would you do? Past and future Mixing!?

About 3 months ago I had a party invited 2 different groups of friends a girl came that I havn't seen for 7 years and my friend at the time came and they got it on.. she hasnt seen me since she hocked up with my friend and know she is mixing with people that i have a past with and i havn't told her why i don't talk to them and i dont like them going out as she hasnt bothered with me since and she is with people from my past when i havn't seen her for 7 years.. i hope this makes since i need advice