FWB issues

I got "together" with my FWB in February. About 3 weeks ago I found out about another FWB he made mid April. Through talking to her we realized he hadn't told either of us about the other. She's a freshman (her name is Lauren). Might I add she is 15 and the age of consent in Michigan is 16 (Me and him are both 17). Anyways I was a bit unsure at first but it all got worked out sorta. She recently told me she didn't want to have sex and was considering ending it soon because she seemed in over her head. Today I found out that they did have sex 3 days ago. Neither of them felt the need to tell me about this, and I had sex with him yesterday. I will be going to get tested and whatnot. I found out they had sex through Lauren's best friend because Lauren lied and said that she told me so her friend talked about it with me not realizing I didn't know. Lauren's mom found out about them and forced Lauren to end it. So ladies, how do I deal. I will not tell Lauren or the FWB that I know what happened. What would you do?