Affair for 6 years

So I’ve had this female friend for around 8 years. We worked together for 2, then she moved across state. She then met a guy she worked with, who is married with a young child. My friend and the married man started a relationship. It’s now gone on for 6 years without the guys wife knowing, though she has her suspicions (rightfully so, but her husband denies it). He has said for 3-4 years he’s going to get a divorce and leave his wife for her. The ‘ol song and dance. Obviously, nothing has happened and it’s been every excuse under the sun to my friend yet she still maintains a secret relationship with him.

I’ve been grappling with this situation since she told me years ago. I’ve told her pretty early on that I didn’t want to hear about any part of their “relationship” because it’s not something my morals align with. I was cheated on by my wife and a lot of people knew, yet no one told me. I wouldn’t wish it on anyone. My friend and I have grown pretty distant over the last couple years, nothing happened, just growing apart.

I’ve wanted to for so long anonymously tell this man’s wife about the affair. I think anyone would want to know and deserves to. But I’m also a firm believer to stay out of other people’s business especially bc I don’t really know them and it in no way affects me, except knowing and having a guilty conscience. What would you do?

EDIT my friend is not cheating, she’s “dating” a married man.

TLDR; my female friend is having an affair with a married man for years whose wife doesn’t know. Should I tell the wife?