Why does my boyfriend want to be friends with a girl he used to have feelings for?

Before my boyfriend of 6 months and I started dating, he had feelings for this girl. I don't know all the details, but I guess he really liked her at one point and she was just leading him on. She was using him for things he would buy her and places he would take her. She ended up having a baby by this other guy. Anyway, he recently got a new phone for his birthday, so I was checking it out and going through it (not sneakily, he was right next to me and besides that, we're both very open with our phones, as we have nothing to hide). And I seen a text from her wishing him a happy birthday and him saying thank you. I asked why she's texting him and he said he doesn't know, she just remembered. Mhmmmm. So I gave him "the look" lol The next time I saw his phone he had deleted her contact and the message thread. That was the end of May. Just yesterday I was talking about picking up weed for us, from a guy I used to go to school with. We used to be friends, but rarely talk anymore unless I'm buying something. Nothing has ever happened between us, we've always just been friends. So my boyfriend tried to compare us and he asked why he can't be friends with this girl? He said he told her that he has a girlfriend and she said "ok, cool". I really don't even know what to think. Does he really think it's harmless? Or is he just stupid? Lol. Isn't it common sense while in a relationship, you don't have contact with or become "friends" with someone you used to have feelings for? Or am I overreacting? I know my boyfriend is an honest guy for the most part (as you never really know a person until you know them) and I trust him. But it's this girl I don't trust, as she's already used him and hurt him once. Even though she has a baby with another guy, some people don't respect relationships or boundaries. I don't want him to end up in a compromising situation because I will leave him if I feel I can't trust him anymore. Thoughts, comments, concerns, advice?
Why does my boyfriend want to be friends with a girl he used to have feelings for?
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