Take down if not allowed

So I’ve had this friend of nearly 10 years we went through high school together and went to each other’s graduations. Went to her Deb have been through everything. I have helped her through every heart ache and any money problem she has had including $400 only couple weeks ago and I’ve got nothing. And for the past year and a half I have been with my partner and it is quite serious we live together and are thinking about kids. Now my ‘friend’ (B) I recently introduced to my partners and I’s group of friends and she’s slept with two of the people already and it hasn’t even been 2 months. And I have found out B has set a goal to sleep with every person of the group including my boyfriend. How would you deal with this situation. I’ve been pretending I don’t know anything but it’s killing me when I see photos of her or when she messages me I don’t know what to do and I really need help please and tell me I’m not overreack