I want to tell man's wife of affair, how do I build the evidence?

[Somersettler]

My best friend (F47) has been having an affair with a married man (M51) for 7 years. He has ruined her life while she has waited for him to leave his wife. My friend has neglected her family and friends in favour of him, and has never been interested in any other serious relationships with other men because of him. She's lonely, and turned to drink to ease her unhappiness. I think she's been unhappy for years because of him.

He has been stringing her along with excuses as to why now is not the right time to leave his wife, says he will eventually, says his relationship with his wife is dead. All the usual stuff. Every 6 months or so she would find the strength to try to end the relationship. Usually when he broke her heart letting her down, not being there for her, or generally treating her like the bit on the side that she has always been. But every time she's tried to get rid of him, he has come crawling back with gifts, and nights away and has always managed to keep her hooked on him. I hate him for what he's doing to her.

Tragically my friend has recently been diagnosed with terminal cancer. My grief and sadness has turned to anger at how he's made her so unhappy for so long. He won't be there for her when she needs him, he's not going to be holding her hand when she's going through treatment.

I want revenge. I won't do anything to upset my friend but if my friend doesn't make it, I absolutely intend to tell his wife. He's got away with it for far too long, and he's taking the piss out of his poor wife. He doesn't get to go back to normal when my friend passes away. I want to destroy him.

The problem is I don't have any proof of their affair. If I tell his wife, he can lie his, way out of it.

So I need to start gathering evidence. What proof do I need and how do I gather it?

That’s a good idea. “Give him a sickener”

If I was the wife, in this situation, I’d want to know. I’d feel like I’d been living a lie and, of course, I’d worry about STD’s so would want to be checked. He is highly likely to do it again, with someone else, further down the line. Having his cake and eating it!

No, but how many do you know. Nobody is saying she would be chilled out about it. You’ve missed the point. She may well be prepared to turn a blind eye. And of course finding out will be hard but she might not even be that surprised. Seven years is a long time!

You are completely right @Gemtastic

The go to response for ow is always "she may be turning a blind eye'

Wishful thinking on many occasions to ease guilt and an excuse that many cheating men promote to their affair partners. "Oh the wife knows but we don't openenly state it"

All part of the ongoing abuse by both parties to abuse an innocent woman.

Many wives stuck at home doing the childcare, the housekeeping and the grunt work whilst another women gets the time, attention and in many cases financial benefits for lacking in morals and being the play thing for a man.

I've met NO woman who happily turned a blind eye, telling this woman could change the dynamics of her marriage, give her some perspective, give back her autonomy and some power balance back.
It's cruel to not tell her.

These rediculous responses about female friendship excusing why this wife should not be told, tell her, if spite and anger are your motivations, so be it.

It's not the friend's secret to tell.

Whatever the OP's reasons for doing this (and IMO they are in the gutter), it is clear that she is going to drop a bomb on this woman and walk away.
The people whose lives she is going to destroy, are the wife and family of this man, all for revenge that the "friend" didn't want.

I do know some people who had "arrangements" in their marriages. But they made knowing choices to stay in this type of marriage. I think after 7 years and so many others knowing about the affair, the wife knows and chose to stay. OP said it's common knowledge.

I also think intent can matter. The only intent here by OP is premeditated malice. They can't change anything their friend did to her own family and friends. Her friend put this man over them. Her friend chose to drink to cope with her choices. Using hurting the innocent bystander in hope of hurting the affair partner is destructive behaviour that leaves a ton of collateral damage.

Stay out of it. Not your circus, not your clowns. Get on with your own life.

Do nothing. It is absolutely none of your business.