I shoot all three cartridges, and like them all. NO bad choices for your intended use if effect on game is the only criteria.
The BC Bison hunting regs show a minimum of
"a centrefire rifle and ammunition constructed with a 175 grain or larger bullet which retains 2 712 joules (2 000 foot pounds) or more energy at 100 metres"
So any of your choices will be legal, and effective, but a couple of factors put the 30-06 ahead of the other two.
The 7x64 is very very similar in field performance to her 6.5, just not a big enough step up in bone breaking power to make the move worthwhile IMHO. And it's a good thing that you handload, because Norma, for instance does not load a bullet heavier than 170 grains. I love the cartridge, but it just doesn't give her much advantage with bears or bison.
The 8x57IS is good and a functional equivalent to the 30-06 in nearly all respects. It is a fun cartridge too and a little different than the common stuff used by most. You would have to avoid any American made factory loads and American data, because 8x57 "Mauser" SAAMI data and loads are low powered and intended to allow interchangeability in the old & obsolete 8x57I ( .318" dia.) rifles. CIP data and factory loads from Europe are the only loads worth considering. She could use SAAMI low pressure loads for practise and deer hunting I suppose.
Or just get her a 30-06. Full power loads are adequate for bison, and it is easy enough to handload or buy light loads for deer and practise. A lady friend of mine recently purchased a Tikka T3 light 30-06 which I set up and sighted in for her. Very accurate, easy to handle, but recoil was a tiny bit sharp. You can't get more power for nothing.
Good luck and happy hunting!
