Hello Ridge Runner, I think that the key to this is the choice of bullet you use. If you go down to 30-30, l recommend the Cutting Edge bullets 130 gr Lever Gun Raptor and load it to high velocities. My grandson used that bullet for plains game in Africa. It kills well.
If you go for a mild load in a 458 WM, which I think is a good idea for bear, as it will kill quickly and just have a slow push of a recoil, I recommend Cutting Edge Bullets Raptors in the 250 grain range at 1,600 fps to 2,000 fps. ( Check the required twist rate on the Cutting Edge Bullet website or phone Cutting edge bullets about it. Any monolithic bullet is longer for weight than a traditional cup and core bullet and may need a faster twist rate to stabilize at lower velocity.)
I promise you that 250gr 458 bullet at 2,000 will kill a bear quickly, about like a .375WM. It's the bullet diameter that makes a big difference not just the velocity.
When using a big bore rifle, it's always fun and productive to have her practice a bit with a mild load, using the cheapest lightest bullet that you can get and load it way down. Don't let her shoot it off of a bench. Then get some one to load a test 20 heavier hunting loads for her. ( In this case 1,900 + fps. and don't have her shoot them off a bench. Thats how I always got my kids going on a big rifle.)
Also, most people who have not used the Cutting Edge Bullet #13 solid will tell you not to use a solid bullet at all. The CEB #13 is different. It penetrates deep and straight and leaves a big non-collapsing wound channel. You may get big entrance and exit hole for a blood trail but it will be a short one. ( I have used them on cape buffalo in a 375WM.)
Many, many bears, big and small have been knocked down with a low velocity 45-70's using a hard cast wide flat nosed bullets. Now-a-days many cape buffalo ( 1500 lbs. - 1800 lbs.) are being killed by one shot with a 45-70 with the right bullets.
Enjoy your hunt. Brian