Saying a .243 is minimum for a deer is a personal opinion, not a fact, a law or a caliber higher on the ethics ladder we all reside on. Each person has to weigh their abilities and make the decision for themselves whether the caliber is sufficient given their own hunting conditions.
Yes, we've all seen these threads where perceived marginal calibers are usually a bone of contention whether they should be used or not.
My typical reply is that the smaller caliber being asked about, is usually good enough for deer, but only broadside shots in the heart/lung area should be taken. The scapula, ie, shoulder blade and the large front leg bone are like armor to the deer and could cause a wounded animal if the hit is marginal.
And yes, like many others here, I have taken many deer with smaller calibers with no problems what so ever. My favorite smaller caliber for deer is my Remington 700 .222.
I would have no problems having a young person use a .223 for deer if I can teach them, through a lot of practice, and they can prove to me they have the marksmanship to use the gun. I would also teach them about waiting for the right shot and being patient or they should pass it up. I would teach them to wait for shots within a certain distance as well.
If after you reviewed more information on the use of the .223 for deer and feel it's not an adequate caliber, the .243 can be purchased in bullet weights of 75 or 80gr which may give a lighter recoil.(I use 100gr) In fact, if you have a .243 or the use of one, have her shoot it and ask what she thinks of it.
Something to think about.
iSnipe