Most Helpful Guy
She probably already does. Adults are weird. Sometimes they lie to themselves about things coz they can't face the consequences. They'd rather just ignore it. It's called denial. It's a cowards way out but I suppose it keeps them sane. I'd stay well clear of it. She'll end up hating you if you force her to face up to something she wants to avoid1
Most Helpful Girl
If it was me, I would want to know. If it was my mom, she would want to know but she is a tough woman and we would rather face the truth. As I seen mentioned in comments, some cannot handle the truth. If your mom is happy, if your dad treats her well, probably best to not take it to her. If you can talk to your dad just to ease your own conscience then do it.
One of my friends found out her dad was cheating, her dad was a jerk, mom wasn't happy, she told her mom, they divorced and now her mom is happier than ever.
My ex's dad was great to his wife except he also cheated. He treated her well, still kissed her cheek, but she was very unhappy. She loved him dearly but he was dinking another woman, she knew it and she swallowed her pain all those years until the kids moved out and they divorced. Now she's a very happy and much healthier woman.
I know a lot of guys are like "stay out of it, it's not your business" but on the other hand, a lot of parents unhappily stay together because of the kids.
There is no easy way to handle this. It's like a bomb, some day it will blow up and make a mess.1