The caliber is less important that how the rifle fits her. Remember women have less upper body strength and more padding on their upper chest.
My wife started out with a 22" 6.5x55 with a cut down stock and had issues shooting it any way but off a rest. She found a 7mm-08 Model Seven Youth with an 18.5" barrel and just flat out kills anything she shoots at. Funny thing is she finds full size short action Model 70 Featherweights also work. Winchester now makes that model in a compact that might also work.
Just let your wife handle various models until she finds the one that works. If you can, find an experienced female hunter to accompany you and her gun shopping.
