Many couples are locked in loveless marriages borne out of economic necessity and/or a desire to provide an outward display of togetherness and stability while their dc are dependent.
However, in the absence of coercion, there is no way that one person can make another do anything they don't want to do and your friend ended her affair with this particular married man of her own volition.
The fact that she found it necessary to canvass opinions suggests that she was not entirely certain that her feelings were reciprocated
Even had she kept the faith for 5 long years of waiting until he made good on his promises, there's no guarantee that the outcome would have been successful from her point of view.
In the intervening years he or she may have met significant others, or he may have reconciled with his wife. It may be that he was lying all along, and it may have been that it was only after yet another of his affairs that his wife filed for divorce and the marital home was sold.
Perhaps he is/was a serial adulterer and his wife was counting the days until their dc became fully independent? Maybe your friend would not have been enough to keep him from straying had they formed a live-in relationship?
In short, there are too many variables in this situation and, instead of feeling awful for giving her the benefit of your sensible advice when asked, you should take the view that some things happen for a reason and the reason could be that your friend may not have been able to withstand years of lonely Christmases and other celebrations while conducting a clandestine relationship with a mm.
Similarly, some things are meant to be - and your friend's continuing relationship with this mm wasn't, and isn't, one of them.