How should you feel if a friend cheats on her partner?

I didn't want to believe. I didn't think she was capable of it. I had no love for her partner; he was a deadbeat and a loser, but getting a new one less than a month after isn't a coincidence. She was miserable, but I told her to stay because I didn't want her to make a mistake. Now she has and I don't know what to make of it. Was it really a mistake ? And even if it was, can I honestly look at her the same way?