If she's been hanging out with your husband and talking a lot with him before her separation, I do think you might be overreacting. Then again, women who get out of a very long relationship, they are on the prowl. If this is sudden, it is something to be wary of.
Also, you need to tell your husband to respect you. If you're uncomfortable with them hanging out alone, he should respect your wishes and not hang out with her alone. He's not single, he's not in HS, it's not the old days anymore. He's married, settled down, and has responsibilities. Then your husband NEEDS to talk to her. He needs to be the one to say, "There's a line, do not cross this line."
I wouldn't go too far, like saying they can't be friends. Just tell your husband to tell her there's a line and if she tries crossing it, that she's gone. He needs to prove to you he said it also. Whether he said it in front of you or through texting. Some men just say, "Yeah, yeah" and not do it. Good luck.
And I agree with the other poster, start getting involved in the things he is interested in. Go on dates, go goof off, and act young again. Married couples need to keep up the spark, so get creative. Men fall over when their woman joins them in their activities.