What powder? How big/old is she? I don't think you could go wrong with any 60 gr bullet and suggested or starting powder charge. I'm sure you could even reduce that a grain or two if you liked.

If she is big enough and strong enough to actually handle the rifle, I wouldn't worry about what little recoil she will get. The thing that will actually be the most offensive to her is the noise. The 243 seems to have a sharp pitch that just goes through a lot of people's ears like a pointed stick. Some people flinch more from noise than recoil. Obviously make dang sure her ears are protected and you can probably forget the rest of the things you're worried about.

I assume she has shot .22s and understands basically what to expect????