I just watched "He's just not that into you", a couple nights ago.. and your situation fits not EXACTLY, but closely related to this certain character in the movie haha
Anyways, you did your part. You don't have any feelings for her, yet she persists. She was half buzzed that night, and hopefully she had forgotten by the next morning. If she still wants to go out for coffee, follow the way of Tao, and cut all contact from her. You already told her how you felt, and nothing is going to change your decision because what's done is done. Highschool feelings are over. If you fall into her trap, that means that you're weak, and it's just going to make matters worse with the husband. (divorce, kids, you know what.)
Another alternative solution, is to tell the husband straight up what you're feeling. He may be disappointed, but better knowing than not knowing at all. That way, he can fix the relationship he's dealing with the wife, and might possibly go back to love between those two.
Just whatever you do, don't fall into her trap.
__________________ Feedback of 40-0-0 -kT: "No man get a ruler and look, 12cm is long!"