Do you trust your Mom? Will your Mom's ever instinct be to keep you out of it?
"Hey Mom, we need to talk about something. It's not something easy to talk about, or that I want to talk about, and I need you to promise me that you will never tell a soul we had this talk. Please. As your son, I'm asking for your word on that.
So first, the most awkward question ever to ask your parent. Do you and dad have an open relationship, or any kind of arrangement, where he might enjoy the company of another woman?"
If yes "Ok, good talk."
If no "I don't want to get in the middle of this, and I don't want to ruin my relationship with my father. So you didn't get any of this information from me, [explain what you know, and where to look for any evidence]. I told you because you're my Mom, and I love you, and I felt like this is information anybody in your situation ought to have. But please return the favor, and don't let Dad even suspect I'm the one that tipped you off on this."
Then comfort her however you can, and bow out of the whole mess.
And remember, whatever happens next happened because your father betrayed your mom, not because of anything you did. You'd tell your buddy if his girlfriend was cheating on him, so you extended that same courtesy to the woman who bore and raised you. That's all.