Sounds earth shattering for you, as anyone would expect a discovery like that, to be. Adults, do make mistakes that come with consequences. It's tough being thrust head first into an adult universe. It's probably better, to somehow confront your mom, and tell her how much this is hurting you.

Be prepared, as many people do this, for her to turn it around to the fact that you invaded her privacy. Just apologize for that err in judgment, and tell her, you'd like to keep to the topic, at hand.

It's easy, to point fingers, and vilify her, for what you may perceive, as shameful behavior. This is just symptomatic, that there's something not quite right about you mom's and dad's relationship, pretty much. Maybe, she's been unhappy, for years and years and this is newfound excitement, for her? Doesn't make it appropriate, but at some point, your parents are two human beings travelling this journey we call life.

You may want to consider some one on one counseling, to deal with the grief of losing the reality of a happy marriage, all these years between your parents.