Some good advice in the replies so far. Have her practice with both rifles, and let her shoot the rifle that she is the most comfortable with. Also have her practice a lot (like several boxes of ammo) with the rifle that she will be using. Spread the practice out over several months and practice shooting with field positions that she would likely use on her hunt.

I've killed a couple of black bears, spot and stalk, one bullet each. One bear with my 45 acp model 1911, and the other with a .44 mag. Neither bear ran very far after being hit, and both of those pistols have a fraction of the power of either a .308 Win or .270 Win.