I don't call myself an anarcho-communist, but I'll answer the question as if you were asking how an anarchist would deal with the situation.
It's all contextual, right? I don't think there's a one-size-fits-all solution. Instead, I think an anarchist would ask themselves fundamental questions about the situation to better understand it in order to make a decision. Why was the person cheating? What were the conditions that led to that happening? What was the relationship like beforehand?
An anarchist would keep in mind that peoples' behavior is begotten by the conditions that led them to where they are (and vice versa at the same time). Cheating may be a result of some unfulfilled need, a response to a perceived sleight, or maybe even just an individuals' desire for passion.
Also an anarchist would probably ask: Why marriage? It's a relationship based on possession and control, sanctified by the state in order to maintain a stable family unit that serves as a foundation for much of the labor that capitalism requires.