Messy situation please help!

So my boyfriends mom invited his ex's mom to his graduation party this weekend. He told me that she never asked him if it was okay or not and his mom was inviting her to be "nice" since at one point lived with them for a few weeks. Him and his ex were only together for half a year or so. Always on and off. Last night she posted on Twitter that she would be coming to his grad party. I talked to him about it but feel like he doesn't care. I'm not at all threatened by his ex😂 she doesn't have nothing on me. But at the same time it's more of a respect thing and I don't understand why he didn't tell her she couldn't come? What should I do or how should I even react if she is there? I think my boyfriend likes the attention from girls and that's why he doesn't say anything. Really worries me what he does when he isn't with me. Especially since we only see each other once every two weeks.