I can't speak to your A's personal recovery.
My thought is you can't unwind the clock. You read it. I don't think you should have personally. It was a mistake on your part but you did it and now you have information. You can't erase it and this event is now part of your marriage in a way it was not before.
It could very well be this was some drunken fool escapade he barely participated, never repeated, never seen the girl again blah blah. If that were the case he may never make an amends. I'm actually OK with that. Some times fessing up to something like that causes so much damage we hurt another to off load a guilty conscious, which is not fair.
But you know. He doesn't know you know. There is now dishonesty and secrets on both your parts. That changes the take on amends in my opinion. Of course he should discuss it with you if you *know* about it.
I'm going to take a lone ranger stance here and since you know it I think you should bring it up. He will never ever win with you no matter how stellar his recovery is if he chooses to not come clean with this (which I think would be OK in some circumstances if you did not know) because you are not going to move past it (and I wouldn't either) without working through it with him.
You have lost trust in him because of this. He will lose some trust in you because you breached his privacy so severely. It is something you'll both have to work through if you want to move on but you can't if you are each harboring secrets.
I'm sorry. It is a terrible spot to be in. Alcoholism results in so much damage to people's lives
