Let's amend that - anyone hunting in grizz country needs their heads screwed on straight. The bears know what a dinner bell sounds like. Mark had too little gun (I think we can agree that a 10mm Glock is light for stopping a charging grizzly bear), wasn't cautious enough, and trusted his client too much. And while he was a friend, he died because he made a series of poor decisions. One of those was the gun.
If you knew for a fact that you would end up in that situation, you'd want all the gun you could get - likely something in the .458 to .577 range. Since no one knows what the future holds and you have to carry the thing and likely want some reach as well as a stopper, something like a light .375 is a reasonable compromise.
