That is heart breaking if true, but it is not a circumstance that does not give both of them something out of it. Let her sleep with that ex she pinning for, but also leave her over there for that ex to take care from then on out. No need to waste time putting in divorce papers, she is passing on anyway. That way, she can be with the man she was always really pinning for, and you can get rid of the woman who only saw you as a means to an end.