On the other side of the coin, even if she was seperated, seperation is a time for cooling down and ultimate reconcilliation. Unfortunately, too many people use it as a time to stick their toe in the water of romance. Well gee whiz, suddenly someone is paying more attention to me than hubby did. Isn't that a common theme in dating, putting your best foot forward always. IF he interferred with the reconcilliation process, despite her assurances of no hope, then he was IMHO and I believe by scripture in error.
If you want to talk scripture or canon law - that's another subject.
Just bear in mind that one day, we will all be called to account for our actions and decisions; most people try to do the thing that/make the decision that is best for them and their dependents, and, I'm not going to "Monday morning quarterback" their rationale *when* I know not "thing one" about the circumstances at the time.
If they break a law, bring the matter before a Grand Jury, let the GJ indict them, let the prosecutor charge them, put me on the jury to judge the facts as you try them - that's as far as I'm willing to go ...