Re: Wife does not see this as bad
10 years of what could have been a steady dopamine fix for her, for all you know. And you were none the wiser.
From my perspective, both she and your "friend" were equally at fault and responsible. He definitely should have left you two alone when you got back together, but she should have told him to stay away as well. Neither happened. Thing is, you're choosing to remain with one of them for life, so I'd be very clear as to what standards you expect there to be in terms of transparency for the duration of your marriage.
All the best.
"If you deliberately plan on being less than you are capable of being, then I warn you that you'll be unhappy for the rest of your life."
~ Abraham Maslow