It might be too late to do this now, but anyway, here's what I'd do. As millipede suggests, I'd call my husband's bluff. Is it too late to back off a little? I think you need more velvet glove than iron fist.
If possible, what I'd do is be very complementary to my husband about this woman, saying as this as this woman seems so supportive to you and so friendly, lets all meet up to say thank you to her. After all you know her from university, so you can say you'd really like to renew the friendship.
Your husband is portraying himself as Mr Nice guy, totally on the level, so how can he refuse? He might even be a bit relieved if he feels this woman's attentions are getting too much.
If he ducks out of making this arrangement, ask why exactly? Say you can't see a reason for permanently excluding you from their meetings. If you're not allowed to see make friends with the woman, and she objects to ever meeting you, then she's actually being very hostile towards you. What have you ever done to upset her? you don't like the idea of dh making friends with someone who actively dislikes his you, his wife. She is overstepping the mark and you want dh to stop this now. Don't even mention you are worried about them having an affair - not necessary and why flatter you husband's ego even more?
Your dh may well be very flattered by this woman's attention but there's nothing huge to indicate there's any more to it. But I think you must take this seriously now and do something.