something off about this post it seems but assuming it's genuine, I have a different opinion than most. do you love your husband? is he happy hanging out with his ex girlfriend or talking with her or whatever they're doing? I love my kids for example and I wish for their happiness EVEN WHEN it doesn't involve me. the same with my wife and vice versa. she's dressed me for dates. we are 100% honest about who we are and what makes us happy. that's true love. jealousy and control and selfishness is not love imho. if you love him support his happiness. if you don't love him than you shouldn't have been with him anyway. I assume you've been together long enough to hopefully know him as a person. he's still that person.

frankly, if you show this much love and understanding and offer to him that you're there for him when he needs you as his true loving friend, he's likely to take you up on it and your kindness is likely to drive his ex bonkers unless she loves him and you all become great friends

good luck