Ex-best friend?

So, my boyfriend and I have been dating for 5 or so months. Only now it’s gotten to me about his best friend. His best friend, is his ex. I know he was pretty mad in love with her, and he was really hurt after they ended. I know he’s said that he was too nice to become friends with her again, but they are anyway. But I still can’t get comfortable now with the fact that his ex, is his best friend. But saying this, I really don’t want to get in the way of him and his best friend, I know he is close with her and I don’t want to get in the way of that. I do trust him, I really do and we’re mad in love. But I’m still uncomfortable about it. I don’t exactly want to worry him by telling him because it might ruin things. What should I do?

Note: she’s got a boyfriend too