It's pretty dated, I guess, but this 1980's Alaskan Forest Service study makes for some interesting reading. It's probably been posted on here before, but it was my first time reading it.
Clearly, some of you folks here know your stuff...interesting bear thread indeed.
I don't know a heckuva lot about calibres much larger than 30 - the study mentions a round called a .460 Weatherby Magnum, is that powerful? ;-)
Lol...of course the recoil might send her back south. Recoil is addressed in the study as well. First place for overall destructiveness went to something called a .458 Winchester Magnum.
And, of course, there is more to the equation than just the ammo: firearm weight, manageability, portability etc.
All in all, what I got out of it was, handguns, even the .44 mag, are not particularly effective, 12 gauge with slugs are good, but buckshot is not, and once the bear is down, keep shooting!
At any rate, I agree with those that have said it should be the least of her worries, attacks being so rare and all...plus having a guard as well.
http://www.fs.fed.us/pnw/pubs/gtr152.pdf