OK, maybe I'm being out of line here but... Wouldn't the better choice be for find a place where she can "test fire" a variety and let her decide? While your concern the .308 is duly noted I have to ask if that is your determination or hers?
Keep in mind that I am 5' 6 and lately have managed to get my weight up to 120 pounds. I'm told it's painful for bystanders to watch me fire my 303 Enfield because of my small size. However, I can fire the weapon all day long and have less discomfort than a certain unnamed idiot that shall remain my brother with his 30-06.
Quite frankly I have the same trouble with hubby, he's decided the .303 is "too big" for my size and hands me inferior weapons more "suited" to me. What he doesn't realize in his "oh so practical" determination is that I've been firing that weapon since I was a little girl. The now departed uncle I adored told me long ago that I was big enough to handle the thing and I might as well keep it. At my first 5 round 10 circle string at 100 meters the same uncle told me if I kept it up I'd be "worth riding the river with."
Make sure the situation is broke before you fix it. Things may not be as they seem. I can't help but wonder if the 308 that's "beating her up" is really a comforting reminder of long ago when she share a golden moment with her father. Those are hard to replace with another rifle, unless you manage to insert another moment to it. Which could easily be why she loved your AR.