You know where your feelings stand, but maybe Zack felt something more like an emotional affair on his part. I don't feel it is too fair to think it's just the wife, perhaps but even so you only have what he said to go by, especially since she hasn't spoken for herself about the issue.

I know that I would not really feel comfortable with my husband crying on another womens shoulders, after that is part of the reason I am there, maybe he was leaning on you a litttle to much instead of his wife. There is a point where it becomes unbalanced, and it could very well be him and not her. While you did nothing wrong her feelings may not be out of place either.

It's not your fault the friendship came to an end. I would repsect his words and choice to end it tho. Doing otherwise at this point is more likely to complicate things further. It's hard to loose a friend but it's not all lost. The time you did spend together still has value. I would try to look at it that way were it me.

Maybe things will change but I would not initiate at this point.