From personal experience - and I realize everyone's experience is different, and everyone's expectations from life and relationships are different - once trust has been broken, it stays broken. And sometimes it's better for all parties if it stays that way, and everyone moves on.

I wasn't recommending a marriage counselor to try to save the marriage, but so that both parties can be honest with each other with a mediator who can make sure it is a proactive discussion and both parties can feel supported. Seeking the advice or help of a professional at such a difficult time is warranted whether or not the intention of either party is to try to continue the relationship in a healthy way, but could be helpful in formulating a plan if they decide to end it as well. Approaching things from the angle of an amicable outcome.

Of course, no matter what, there will be broken hearts. I do not envy these people, and absolutely wish them the best and hope they find peace with their decisions.

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Also, I think the poster is asking from a logical standpoint rather than an emotional one, unless I'm wrong (quite possible). And if that's the case, it doesn't matter which party is asking, or whether they are even involved in the relationship. None of us has the answer that will solve this situation, though everyone would like to help. In the end, all we have is opinions, because we have varied experiences, and none of us are in their shoes. I'm interested in other people's experiences, though, because I'm not awesome at relationships, and I know there are some people who can make anything work with enough determination. I guess I'm one of the people who asks "Why?" We only have one life, and it isn't easy, so I have respect for people who stick things out, but also for people who recognize futility and are grown up and decisive about it. Again, from personal experience, I have stayed in things for far longer than I should have, and it ate parts of my life and my heart that I will never get back. But I guess that's why I'm on the therapist's couch and not in her chair :P