I disagree. While yes, if he continues to desire to be "elsewhere" and searching for this passionate love in other places or even wondering it would be a sign to get out, I think it's rather simplistic to say that it's horrible to stay if you have these thoughts. Thoughts, doubts, concerns and such are entirely natural in life and every relationship will run warmer at some times and cooler at others. Many many people do go through this and get past it, rekindling their love for the one they are with. So my point only being is that having the thoughts is one thing and is a sign that SOMETHING must be done but leaving out the ability to reconcile this doubt in his mind just leaves out something that should be considered.

That being said, again something must be done and that is making a choice to either resolve this and get past it with thoughts of staying with her but only if he can reconcile his doubts first, or... the other option of yes, leaving her because continuing in a relationship where you're still looking is unfair to the spouse.