This certainly feels like a tough decision for you at the moment.
Hopefully, you see that from our perspective this is a no-brainer. Thta's because we've seen it over and over and over and over.
Your assessment of OM's thiking is probably spot on.
If I confront her and she chooses him and then finds out she has cancer, she'd have no one to support her emotionally --
Yes she would. She'd have you. She may not want to lean on you immediately, but you would be there to support her, right? You recognize already that OM would be gone in a nanosecond if she has cancer. You won't be. All the more reason to not wait and confront her now so that if the worst comes true, OM will reveal his true colors, as will you.
You do not need to reveal your sources to expose to her. Just confidently express your level of knowledge. She'll know you know.
Whenever you reveal it, be prepared to expose to OM's wife immediately. Otherwise, your wife will warn him and he'll prep his wife that "some nutball has accused me of boinking his wife! Isn't that crazy, honey??"
Do you know how to contact OM's wife?
Another way to look at this regarding showing your source of info is that you probably will HAVE to show solid evidence to OM's wife. If she's an unsuspecting BS she very likely will not believe hearsay that her lovable H is having an affair. Very typical. So, it may be prudent to reveal to her your evidence even though it may get back to your wife via OM. But this is also a valuable data point - indicating that OM wasn't immediately scared off.
Please read the link about exposure in my sig line below.
Also,
She doesn't desire me or love me. She claims it's because she hurts over what she did to our marriage in the first affair -- how she has destroyed me and forever changed me. She can't be intimate with me or desire me because of the pain she feels.
Priceless, in a dark humor sort of way. More proof of the Mothership.