Since it hasn't been mentioned yet, take her to a gun store/show and have her fondle a bunch of different brands and models. Make sure she handels bolt-action, lever action, semi-auto, and pump action (if available). When she finds one that feels right to her, then see what the caliber options are. From there you can discuss with her the relative merits of each caliber and let her choose.
Before you steer her to a small round like the .243 have her try out a few rifles from your and your buddies' collections so she can get an idea as to how different calibers feel in terms of recoil. You might well be surprised at how much recoil she can tolerate.
This is not to put down the .243 as a deer rifle at all. Just that a new hunter is often better off with more than the minimum acceptable cartridge (while some do use .223 for deer, and are quite successful with it, the .243 is generally considered by many people to be the smallest one should go for deer hunting).