With proper load and placement, sure, 6.8 can work. .22 lr can work as well with proper placement, however. You can put down a bison with .22 lr and proper placement. Bear generally range from 100-550 lbs in the US. So your considerations are shot placement and risk to the hunter, regardless of caliber. People have been hurt by bears after shooting them with larger calibers. So a guided hunt would be a foot stalk hunt? How good is Kris with shooting under pressure? How good is Kris with shooting running targets? Will the guide have a gun on the bear as well? More importantly, what does the guide say about using a 6.8?
People do hunt bear with the 6.8 spc. It can work. Preference seems to be for using deep penetrating bullets such as Barnes TTSX.
I think that if I was on the ground, I would want a larger caliber. If I was in a tree/elevated blind, 6.8 would be fine.