I shot my bison this fall (got to figure out how to upload a video) with my 9.3x62 at 37 yards. I was using 286gr Norma bullets (thanks again Ted) and they run about 2350 at the muzzle. My first shot was a tight quartering on shot into the chest cavity. On taking the first shot he reared up and came down with his right leg stiff (classic shot through the brachial plexus). He ran across the trail where I hit him through the shoulder with another shot which stopped him. I then hit him behind the shoulder which knocked him down. That said he did still require a shot in the back of the head to finish things in the fastest manner possible. My second shot broke the shoulder (a helluva bone) and soem ribs before going through the left side lung. The third shot was behind the shoulder. None of them got the heart though I think that the first shot hit the aorta judging by the blood in the chest cavity.
Granted the bullet is a cup and core style and perhaps a premium bullet liek a Partition would have penetrated better (I tried the 286gr TSX when I bought the rifle but couldn't get enough speed out of it 2000fps max), but there's a lot of meat to be driven through to get to the vitals when dealing with a Wood Bison. Sometime you'll need to hit them more than once to keep them from running off into some deep nasty crap that you'll have to drag them out of. They're a big enough job lyign on the side of a trail, you don't want them to disappear into a willow slough or some narrow dry creek bed.
Barring improper bullet constriction, there is not good reason that you shouldn't be able to kill a bison by hitting it in the heart and head with a 30-06. Mind you, there's a lot of "head" that isn't "brain". Maybe your co-worker didn't shoot him particularly well.