4my, tell OW's H straight up. And soon. That his W is involved in an affair with your H, how you discovered it, how long it's been going on, that they work together. He deserves to know she's cheating on him.
Yep -- bingo. Have compassion ("I'm sorry to have to tell you this") but stick to the facts ( " . . . but I thought you should know." Throw in a "I would want to know if the situation were reversed.
Tell her you'll respect her privacy if she doesn't want to discuss it, or you'll share your proof with her if she wants to see it -- her call.
