I too was cheated on by my exhusband. No one told me about it until after we were divorced. Did it make me feel better or give me more justification for the divorce? No, not really. It was only confirming suspicions I already had.

I think deep down, we all know when something is not right with our marriage and we choose to either confront it or ignore it.

Is it right to tell this woman? No, I don't think so. Think about why you want to tell. Is it because you're sure this woman needs to know, or because you know about it and need to "get it out of your system"

So many times, the "teller" thinks they're doing the other person a favor by telling them about their wandering spouse. I'm really not sure that that is such a good position to be in.

Do you remember where "kill the messenger" came from? If you have a friendship with this person, you could be risking losing it.

Just my convoluted two cents.