I don't know what to think about this..

I've been with my boyfriend for a while now and I just can't help but find it strange he talks to this older woman he met in AA/NA in his teens (he is 26 now and I am 22). She's in her mid 50s and tbh she's the one that keeps talking to him mainly, starting convos, and relying on him for emotional support a ton. In a way I kind of feel for her since she's been through a lot (what he has told me and from 2 times I briefly spoke with her). I'm weirded out still since they (her and my bf) had a sexual relationship when he was 17/18 and she could honestly be his grandma or something when it comes to age. I also thought that dynamic was fucked up and I thought that it was assault but he defended her. During that time he was a drug addict and was vulnerable so I still think it was her fault tbh since the age gap is so huge and he was so vulnerable. She keeps texting him and calling him all the time and like 60% of the time he doesn't respond much but it's getting to a point where I don't know how to feel about it anymore. I usually don't have the best judgement so I wanted an opinion on how to deal with this situation. I've told my boyfriend that I thought it was strange for her to always go to him and try to talk to him all the time. He understood and agreed to an extent but it's happening all over again. I also don't think he has come to full terms if he was assaulted or not.