Well you kind of have two immediate options.
1. Say forget it, forget the time I have sober, forget the progress I made, I am drinking. Join your husband drinking. Possibly get into a fight while drinking because, you know, we are usually invincible while drinking so you are blunt in how you feel etc.
2. Dont drink, think about this rationally, stay clear headed, and find a time to really discuss what is going on with your feelings, and the actions of your husband.
Tell him how you are feeling, and how hurt you would be if something were to happen etc. If you trust him, tell him you trust him but you dont fully trust someone you have never met, and thats the biggest thing, you have never met. Possibly meeting this person might put your mind at ease, and then again it might not.
Sometimes guys, and I am one, like attention from the opposite sex outside of their marriage and or relationships, because it makes them feel like they still have it. They still have what makes them attractive to the opposite sex, a self esteem booster. For women, knowing they are still attractive is much easier, as you can see a guys head turn and almost break your neck as you walk buy. We arent the most tactful in what we do. Women are though, women tend not to play their hand right away.
Some questions to ask yourself-
1. When he comes home after seeing her, does he take interest in you? Or does he ignore you, and not even look at you?
2. After he sees her, is he happy, does he come home happy, or is he sad, upset etc?
3. Is he still interested in you as a person, your thoughts your feelings, does he still look at you in a way that makes you feel attractive?
If he ignores you, is sad when he gets home, and doesnt look at you much, then something might be going on or he is at least thinking that he might want to do something... If he is still into you, Id say you dont have much to worry about.
Maybe this helps, maybe it doesnt, but being a guy, Im just trying to tell you how I feel etc. I had a lot of friends that were girls while dating, that sometimes that person who I was dating got jealous. But I would try and reassure them, that I am with that you, not her, I am going home with you.....
Anyways I think a rational discussion, while both are sober might help clear some of this up.