My ex turned out to be my now good friends husband.......?

I dated this guy over 4 years ago. I knew of his wife but didn't actually know her. They seperated and from my understanding at the time were in divorce proceedings. Over 4 years later, I met his wife through a mutual friend. We've become really close and I like her. I feel like I'm betraying her because I've dated him and don't know what to do. They're having marital problems now because of his infidelity...another woman. She asks me for advise all the time. Should I tell her about us or not? What would you do?