Re: what caliber for long range?
A good muzzle brake will tame even the big magnums.
Be sure to get a rifle with a fairly heavy barrel. The weight will help steady the rifle and reduce felt recoil.
You'll need a barrel with a fast enough twist to stabilize the heaviest VLDs with the highest BCs for whatever caliber you choose.
A well designed and proper fitting stock also makes a difference.
And, don't forget to budget for quality optics, range finder, reloading tools, reloading components, etc.
With all of that in mind, my son has been shooting a Remington Sendero 7mm Rem Mag with a KDF brake since he was 13. He's 15 now and just weighs over 100lbs. With that weight rifle/barrel and a brake, he's always enjoyed shooting it extensively. So, anything from there on down is very doable for your wife.
243 and 308 are excellent choices because they perform well and factory ammo is readily available. Both have exellent barrel life.
The 260 Rem is a nice balance between those 2 cartridges. With excellent ballistics and bullet choices, it probably has better long range potential for your situation.
6BR Norma is very accurate to 1k, minimal recoil, and very long barrel life. But, it lacks energy out past 500 yds. ...as does 243. Also, you will either need to handload (which you need to do anyways if you shoot much long range). Or, you can order excellent 6BR Norma ammo online that's made by Lapua.
Best of luck!