I'm going to take a different tactic, especially since this is an interesting post for post #1 for this person.
I wouldn't go all ballistic on either of them. Frankly, while I think women who come on to other women's men are skanks, the bottom line is that your husband is the one who could and should have said "No thanks, I'm married". That would have ended the cyber dalliance, I would imagine.
That being said, here's what I'd do:
I'd tell him I saw this interchange between him and this woman.
I'd tell him quite clearly that I do not like this interchange and it needs to stop now. Try "I don't like this and it needs to stop now".
When he did the usual "We're friends, you're overreacting, it doesn't mean anything" protestations, I'd say "Good. Since it doesn't mean anything it should be quite simple for you to stop it now". Your issue should be with him, and him only. A skank isn't worth your energy.
I'd take a look at our financials, maybe make a little copy of everything that I'd put in a safe place.
I'd pay close attention to his behavior to make sure he really did stop.
I'd ignore the woman...the fact is, your husband made the vow to you, she didn't.
See, here's the thing...there is nothing more important ot ma than my marriage, and I would expect my DH to feel the same way. Therefore, if I told him something made me uncomfortable and I wanted it to stop, I'd expect it to stop. If he told me something made him uncomfortable and he wanted it to stop, I'd stop.
So, in the next few months you will learn many things:
The kind of man your husband is.
How much your husband values your marriage.
How trustworthy (or not) your husband is.
How much you stand to get in a divorce and where the money is.
Good luck...