Get her two identical rifles, one in .223 and the other something sufficient - 30/06, .358 Win, etc. and scope them the same. Buy 1,000 rounds of .223 ammo and have her shoot the .223 until she's completely comfortable with it. With a light caliber, there's no chance she'll develop a flinch, and you can really work on developing form, working the bolt from the shoulder, shooting off sticks, etc.
When it's time for the trip, switch them out. She'll have no fear or issue shooting the larger cal, especially in an actual hunting situation. The rifle will feel and function the same as the .223 that she shot all summer.
