Having never walked in his or his wife's shoes, I am loathe to judge.
I used to be much more critical about these kinds of behaviors. Now that I've been on this earth a while and seen a few things, I have some perspective.
Terminal illness is a bitch to deal with, a huge stressor on not just the patient but the immediate and extended family. I know that my mother urged my dad to remarry as soon as he could. More because she knew what a fuckup parent he was and less because she wanted his happiness. She may have even turned a blind eye to his early start on this assignment.
I personally know couples who for whatever reason have chosen an arrangement in which one partner seeks engagements outside the relationship and the other partner is okay with it.
Who's to say that Mrs. Edwards (or Mrs. Clinton, back in the day) didn't give her full consent, or even encourage it (yes, that is some people's fetish and who am I to judge another person's kink?), for whatever reason? Only they chose to publicly act the ignorant and offended spouse because to do otherwise would heap even more public ridicule and judgement on them than to go along with the traditional monogamy-centric response.
This IMO is just further evidence that Monogamy is a social construct that is ill-fitting for a significant portion of the population. Rather than judge, perhaps we could take a look at it, deconstruct it, evaluate whether it's effect is to benefit - or control - society, and decide whether to abide by it or not, rather than unquestioningly accept it.
On balance, I would much rather have a leader who fucks his associate or his intern, than one who fucks me, the economy, young enlisted men and women, the Constitution, our civil liberties and god knows what else repeatedly in the ass without our consent.