The ESEE 4 is quite the knife, the handle is much thinner than the 5 and 6.
The 5 is a beast, thick and heavy, unless she is cutting herself out of a plane, she won't need that much knife.
The 6 is a great design, their flagship knife, but too long for many. For a "survival" situation, a bigger knife will never trump knowledge and skills.
If she ever needs that knife, to survive, she can do anything with the ESEE 4 that a bigger knife would do. I mean, how strong can she be?
I think of my ESEE 4 as a great knife, not my favorite, but I'd stake my life on it's quality, size and design.
There are plenty of knifes that would work for her, just depends on which feels best in her hands.
The Bark River STS-3 and 5 both have thinner handles, as well as the Highland Special, Woodland Special, and others. They will be more expensive than ESEE.
Becker is also coming out with some medium sized knives, then there's always the smaller K-Bars.