The title sounds terrible, but keep reading please

My boyfriend (18 at the time 14) told me that his older sister's friend used to like him and was "friends with benefits" with himwhile she was 18. Already a red flag in my head. This is also my boyfriend's best friend's older sister. My boyfriend had a crush on her since he was little and she started talking to him more when he was 14. She started confiding in him about her boyfriend troubles, then it became a little sexual. He had told me when we first started dating and he stopped paying mind to her she started saying she misses him. That she's still in love with him, etc... She doesn't talk to him much, but when she does, it's just photos of her or photos of her and her boyfriend. It all seems dodgey to me and he is still upset by it. He thinks that it's his fault for even liking her and "starting" the trend of them. So how do I talk to him?