I saw a couple that was doing "retaliatory" cheating (and telling each other about it). When they got through their anger, they decided to call a truce and made peace. With their level of emotional maturity, I doubt it lasted. I don't know if I helped them or prolonged their suffering. It was their decision to come to counseling, so I think it was the help they wanted.
Other clients realize what they really want is "divorce counselling". What's the best way to behave civilly and minimize damage to the kids while we go our separate ways?