Grady
AH enthusiast
So I guess what I'm saying is - pretty hard for you or us to make a decision for her. Let her try a few rifles, combinations of Calibers and stocks ( may have to beg and borrow a few guns, to get a decent cross section) and she can tell you what she likes and what feels good to her. That might narrow it down - if she likes something a bit bigger and heavier ( like a 30-06 0r 30 cal mag), she's set for life and there will be no hard decisions or upgrading ever needed. If she prefers something a little milder, lots of options there too.
This is probably the best piece of advice in all the replies, followed closely by rookhawk's advice (Ladies stock).
The 6.5 CM is a fine caliber, in fact I shot my Kudu with mine, after a not so stellar shot with my bow.
As someone who just took my daughter to RSA, the thing that worked for her, was that she got to choose the rifle (.308) that she wanted to shoot and that made it even more enjoyable for the both of us. I let her try out different ones (6.5CM, 30-06, 25-06, 270, 308, 300WM) While she them all pretty well, she enjoyed and shot the .308 the best and that was her choice. Because of that, she also claimed MY 308 as her own (a small price for me to pay). She is average size for a young woman (17yo, 5'7") and did not need the ladies stock. She shot an Impala, Blesbok, Warthog, Red Hartebeest and Nyala) All only needed one shot. (168gr Barnes TTSX, 43.3 gr Varget)
Bottom line, let her shoot different rifles if you are able, let he make the decision on her primary rifle, it will make the trip that much more enjoyable for her which means you will probably enjoy it more also. Since you said she has/can shoot your 404, she can us that on the bigger animals if her choice is a little light in your opinion.