The difference is whether or not Bob is b*tching because he's on the road for so long or if he's truly feeling fatigued.
If he's b*tching about being on the road for so long, then he has a crappy job, , but he better fly the Plane-2 if he wants to keep the job.
If he's truly fatigued and needs a stand-down, then this is where he mans up, steps up to the plate and point blank says he's tired and needs a break, and offers alternatives.
Now, Bob also may need to have a sit-down with the boss and present a nice brief on crew fatigue and how this may impact the big man's personal safety, the safety of his loved ones, or his company, etc.
You can look at NBAA for help on this as well.
And I would still look elsewhere. Aircraft owners who don't understand fatigue and the risks of fatigued crews need to hear it from multiple people on multiple occasions before they accept it, and in the process they'll fire crews thinking they're whiners... and Bob doesn't want to get caught with his pants down.