Sectional density doesn't mean squat with the amazing selection of bullets we have at our disposal today. Load it with monos, NPTs, Northforks, or some other stout deep penetrating bullet and make sure you knife is sharp
Furthermore, I've found caribou to be the easiest species of the deer family to kill. I'd bet on any given day of the week that a 125 pound whitetail will take more killing than the biggest bull caribou you can find. Moose IME aren't too hard to kill either, they may stand around for a while after you poke them but they usually tip over pretty easy. I used to think otherwise, but I've seen little kids knock over some pretty big bulls in the last few years using 243s and factory ammo.
Get her a good fitting rifle in any of the calibers you've mentioned and keep her in practice ammo. When hunting season rolls around load her up with premium bullets and make sure your knife is sharp.
