I would suggest the next time she rides her "good enough" bike - check to see if her stand over height works on that bike. If the cross bar is at the right height for her then measure it to figure out her stand over height - (or at least get a good idea). That is the best I can figure. Her height is a good indicator but you also need her stand over height. Just a thought.
It sounds like a great present!! Good luck.
Oh, her jeans just mean her waist. Doesn't help you at all. Although, measuring the length of the legs on her jeans might help.