Boyfriend(m23) sees ex girlfriend(f23) at parties and I(f22) need help with how to navigate it

My(f22) boyfriend(m23) was in a 5-year relationship with a girl that he went to school with that ended about four years ago, then he met me, and we've been together for a year and a half. We are long distance, met in college, and he went back home to his home state following his graduation. He is relocating to Texas for work and I am going with him in a couple months -- all just background.

Full transparency, I made some bad decisions and learned information that wasn't mine to know about his relationship. He downplayed their connection (said they weren't friends, only for the group, he didn't really like her) when he mentioned he was "still in contact" with an ex. He didn't mention to me that they texted multiple times a month and sometimes weekly. She texted him in the middle of the night early in our relationship and I thought he was cheating on me so I opened his phone. He wasn't, but she had been asking to see a picture of me and asked about how we met. I was uncomfortable by this and read a LOT further in their texts, where I saw various messages between them; she left my boyfriend for another guy (I was told she cheated) and still texted my boyfriend saying she would always love him, which was just, I think, first-love high school sweetheart breakup stuff, except she continued to text him inappropriately about lingerie she bought, she wanted to know if he was sleeping with people, etc. She also got a crush on one of his/their friends, and was asking him for advice because she was still with the other guy she left him for.

So, I don't really like her that much. I think she's slimy for being interested in other people while dating someone and I dislike her prying for my information. I eventually came clean about going through my boyfriend's phone, after I asked him not to text her and he did it anyway - which is a whole other story and was really problematic, but, to summarize, I met her a couple months ago and he thanked her for being nice to me and made a few other comments that rubbed me the wrong way, but we've gotten through it pretty much.

Last week, he described the whole thing as "an itch" to talk to her sometimes. He said he has since gotten over this "itch" and that he's never had any interest in her since their breakup, and that he texted her when he was mad at me over something, fully knowing he shouldn't have? I don't know. He admits every time it comes up that he was wrong for texting her and he wasn't fully transparent about their friendship, but still doesn't know why he kept her in his life like that. Either way, he never cheated on me, and he says it's all over with her, and everything else in our relationship is fine.

Thing is, I asked him to block her number and delete her contact. He did both without complaint. Yet, he's still going to see her at a 4th of July party. I asked him to not talk to her, but he said he would have no idea how to do that, and I don't really want to start drama on his end. What's a reasonable ask? Is it polite of him to just excuse himself when she walks up to him (cause she always checks in on him and wants to catch up)? Should he say "I can't talk to you"? I feel like that's embarrassing for me and makes me look kind of insane. Is it even appropriate of me to ask him not to talk to her? What advice can I give him to politely avoid her and not stir anything up? It should be a decently sized party and I won't be there for it, but I see him a week later for a trip we're taking.