Should He Tell His Friend’s Wife That His Friend Cheated On Her?

At first, I didn’t quite get what this guy was saying. I thought if he was asking if he should tell his friend that his wife cheated on him. Actually, that’s not it at all:

I’ve [26, M] known my friend, D [25, M] for six years and he’s like a brother to me. D’s a really great guy, he’s very funny, smart and an overall really good friend. We work in the same field and have gotten to work on several projects together, even getting to travel for a few months together for this last fall. We live pretty far away from each other, but make a point to see each other every few months. D has been married to his wife L [27, F] for nearly three years, but they’ve been together since high school. The past year, they’ve been going through a rough patch due to D traveling so much and L having no sex drive.

Usually, when D and I meet up, we’ll often stay at each other’s houses, spend sometime catching up, sometimes help each other out with work, and then usually get drunk together one night. It’s our tradition. This was a few weeks ago, but I visited D when L was out of town for work. We ended up drinking and catching up. That night, D admitted to that while we were traveling together he slept with a girl he met. I had no idea this happened. D is the last person that I would think to do this. He’s always seems to be head over heels for L- even despite going through a rough patch. He then proceeded to tell me that he really loves L, but regrets marrying so young since it never gave him a chance to date anyone else and he feels like he’s missing out on his twenties. He still wants to remain married to L and plans to move forward without telling her he cheated. He feels he can fix instead by being a better husband going forward.

I’m also married and love my wife with all of my heart. While I would never cheat on her, if I were to do so, I would feel obligated to tell her. It feels wrong to not tell L what happened but at the same time, it’s not my marriage and I would be betraying D and possibly ruining his marriage (and could hurt his career since he’s relatively well known in our line of work if it were to get out). Anyone I could talk to about this could potentially cause it to get back to D or L, so I’m just going to post here. Anyone else been in this situation? I really need advice. Would I be overstepping? Should I tell L or should I just mind my own business? I’m completely at a loss on what to do.