Hi Lucy, sorry to read of your plight, and welcome to the forum. It's a good place to be.
My H also had/is having? an A with a fragile and rather dysfunctional woman. He has been the OM in their R for a year. Their R is rocky at best, but he still hangs on in there. He told me he wanted to help her get out of her current R with a controlling guy (which also started as an A incidentally.)
There's not too much you can do for your H right now. And it may well be a while before he wakes up and starts to look at what he is doing and the damage to himself and others. Also, I would protect yourself and have no contact with her if she is a bit bonkers. As someone else posted recently - she is a wormette who is not worthy of your wisdom.
Protect yourself as best you can financially and start moving forwards from there. These sitches tend to take a good while to resolve. Can you tell us some more about your marriage? Why do you think your R became vulnerable to him having an A?
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus