My (21F) bf's (26M) childhoodfriend(31F) is more than just a friend

Soooo my bf, let's call him Andy and I have been together for the past 2 years. Our relationship has been truly wonderful. He treats me in a way that makes me forget my slighy irrational dislike for men, going above and beyond to quench my every need. We've had a few rocky paths where I initiated brief breaks to gather myself and assess the situations. In one of those breaks he decided to drunk text an old "friend" while he was on vacation about our issues( he was just moving too fast for me, talking about a shared future which scared me off quite a bit) and they ended up turning on disappearing messages and remenice about the times they had messed about, four years ago, quite explicit stuff as he's told me and she basically said I didn't deserve him. Now he did tell me about that as soon as we were on good terms again and I just shrugged it off because I tried to be mature about it and appreciate his honesty and his regret( I took a mental note). But her name, Tanja, kept on popping up in my head. They've been in communication on and off since we've been together and they have a past. They grew up together and were like "cousins"/childhood friends. They messed around last about 4 years back while she was in a relationship herself so I can't fully blame her for thinking he'd be down to eventually cheat even though he was with me, I mean he obviously didn't care that she was in a relationship when they were intimate either. I read through their recent chats a few days ago, with his consent and it was more text than the flimsy screenshots he'd sent me (unprovoked btw) making it seem as though she was a crazy woman who wanted him or sum. I actually messaged her, wanting her side because I can't fully trust the "crazy ex" narrative, which is something I wouldn't have ever done and we had a call. She was nice enough to send me screenshots dating back to the incident of his drunk attention seeking and I also recorded the phone call because I'm not about to be gaslit. She talked about him being the one always flirting etc. that it was a common thing in their "friendship" and she apologized for having entertained him for so long. She gave (kinda phony) wellwishes, wanting the best for us even though she'd never consider him a suitable match... He on the other hand had told me how she wanted to be introduced to me so they could mess around behind my back so I didn't suspect anything. That's when he apparently drew the line and told her off or whatever...Fast forward we're not really on talking terms, we went our ways quite dramatically with high emotions on his side and with two dissapointment tears on my side because his friends knew about him texting her during that vacation and didn't seem to see anything wrong with that. Now I went all in and texted his mother, politely about their friendship since he's been denying that they were ever in any way friends and his mother told me they were raised closely and had seen eachother a few times even after they moved...