Honestly it comes down to what she'll be hunting and the range she'll be hunting at. I find if I'm going to go hunting someplace other than near my place I make sure I use a caliber that's readily available too where I'm hunting. I'd hate to lose my ammo somehow and then be stuck checking every store trying to find a difficult round.
243 Win - Readily available everywhere, good for up to whitetail, pretty flat shooting and light recoil
257 Roberts - Selective availability, pretty much a good match to 243 with a hair more energy if you use +P
25-06 Rem - Selective availability, I'd consider this is the smallest round I'd consider for anything larger than a whitetail.
6.5 Creedmoor - Good availability most places but not everywhere. Capable of larger animals but I wouldn't take moose hunting
260 Rem - Select availability, almost a match for 6.5 creedmoor with a hair more energy and I find brass sources more readily available. After this you need to make sure your wife is recoil tolerant depending on rifle weight.
270 Win - Probably the king of availability next to 30-06 and 308 Win, can take pretty much anything but still wouldn't choose it for moose hunting. I wish they didn't come in 1:10 twist barrels, I'd like to try a 1:9.5 or 1:9 to try really heavy bullets.
7mm-08 - Pretty good availability, a very versatile round and can be loaded to take about anything.
280 Rem - very select availability, a match for 270 knockdown with better bullet availability.
308 Win - available everywhere, can be loaded to take about anything but I find a lot of the ammo that's available is target loads which I wouldn't take on a hunt.
30-06 - available everywhere and with a ton of different hunting loads. There's not an animal in North America that hasn't been taken by one.
