I don't think I would be bothered by it myself, from my own female perspective. Sometimes people reconnect with old high school pals and a romance develops. Your husband must be over his feelings for that person by now and she is interested in his friend, not your husband --- unless you have reason to believe she is using the friend to get to your husband?

Hmm, it does actually sound like it could be awkward if she was very close to your husband for a significant amount of time. It's possible that it won't last, so maybe it's best to keep your opinion to yourself for a while longer and just see how it plays out?