There's no easy way. This is so hard. I'd just call her and say, "Hello, I'm TP7 and I want to discuss an important personal matter between my wife and your husband. I discovered they've been having an affair for...however long it is.
I know how awful this is, I'm sorry to tell you, but I feel you should know. If there's anything you want me to tell you, I'll tell you what I know. You can call me back too if you wish as I know how hard this is."
I'd get right to the heart of it before she can cut you off. She may already have some inkling that this is going on, people often do.
I think that's all you have to say. You can voice call or text but text is easier to pass on, of course. I'd do voice call myself. She really needs to know, for herself and her kids but also.....your wife may not be the only person he's involved with, and there may have been others before. There probably will be others after.
Be prepared for any kind of response, including anger, but once you tell her, that's the limit of your responsibility. You don't need to do anything else unless you both want to. So if she gets angry or upset, just be easy with her...."I know how hard this is how much it hurts, I'm going through this too. I'll let you go now but you can call me at any time." She may yell, she may cry, she may say nothing, she may thank you, she may want all the details, she may want to meet. You can decide at that point, but once you've told her, you don't have to do anything else, IMO. I think it would not be uncommon for her to get very upset but maybe want to talk to you later. Think of how it would be if someone just told you this, especially if you had no inkling. She might not be surprised - she might be grateful even....but I think this is always a shattering thing to discover.
It's a very hard thing to do, but it's the right thing to do. And it will help to end the affair or make it more difficult to continue, on your side.