SO AND FEMALE FRIENDS??

I don't know how to bring this up without causing problems, but I want to took to me SO about a certain female friend. They use to date in high school and broke up because she moved away. Recently he talked to her a few months ago reminiscing about being together. I was greatly hurt, I know that good relationships are hard to let go. But if you want to remain friends I think boundaries are important. I talked to her about it, she said well I have a live in boyfriend and it's not like that. He didn't think it was a problem to look back at memories, i.e. reminisce together. I didn't find it very appropriate at all and told him how I felt. I don't know if he's still talking to her, I don't care if they remain friends. But she's the one pushing boundaries. I just wanted to casual ask if they still talk. He never even mentioned that he was still in contact with her. I found out when she snapped him at 3 in the morning wishing I'm a Happy New Year.

I was taking a back he admitted he should have told me, I just wish he was honest, I want to know if they're in contact still. We are also in our 2nd year of LDR.

Any advice on how to approach would be helpful.

TL;DR SO still talks to ex girlfriend/friend, didn't know about it until after in the relationship. Want to ask if they're still communicating.