She doesn't know whether to Dad about Mum's lover

I recently discovered that my mum is having an affair behind our dad’s back. I am 27 and live at home.
She left her Facebook logged onto my computer and I found racy messages between her and a younger man who works in the same office as hers. I confronted her and she confessed, but she made me swear not to tell my dad.
I feel my dad has a right to know so he could put a stop to it, but I know that if he does find out, it would break up the family. I can't trust my mum any more.
This discovery is making me really miserable. What should I do?
It’s unfortunate that you’ve been put in this difficult position. But as painful as it is, your parents’ marriage is not your responsibility.
This may sound harsh, but the way you found out about this affair was also breaking trust as you had no excuse prying into her private chats.
It’s a big burden you’re carrying, but you need to share your feelings with a couple of your trusted friends. What your mum is up to is not unusual, but it doesn’t make it right.
After you’ve cooled down a bit, talk it over with your mum and let her know you would be relieved if she could put a stop to the affair.
(Bunmi Sofola)