Sometimes if people want out of a relationship they will create circumstances (both consiously and subconsciously) whereby the relationship cant continue. In this circumstance maybe the wife wanted out of the marriage (she was playing around after all) but didn't want to feel guilty about it.
If so, accusing her husband of cheating would be a good way to break up but she would be able to blame him for it thereby avoiding her own guilt.
I'm not saying its right - i'm just putting it forward as a possible explanation. It seems to fit.