Should I allow him to see her?

My boyfriend and I are generally happy together, but recently his old "friend with benefits" has come back into town and I had found texts of him talking with her about hanging out/ when to. It troubled me because he was once attracted to her so what's stopping him this time. I had sat down with him and discussed how it bothered me severely, which brought us to a agreement (he could see her but not alone) but during a party, she was there and they started chatting. She was obviously still attracted to him, maybe it was the overdone laughing when he asked about her life.....I don't know but now I found out even though I had told him it bothered me for him to hang out with her, he's trying to anyways. What should I do? Should I say something again or is that just going to be a waste of time?