Tell her to move to a smaller planet. 
On a long dayhike on a hot summer day I can drink 7-8 liters of water easy, and still lose weight.
I hike and bike and backcountry ski all the time. I have tree trunk legs. But it takes a long time (and the proper genetics) to get that way. There is simply no way to build leg muscles like that in a two week span. And while hiking is a great workout and will eventually build your legs, it doesn’t really compare to dedicated weight training (squats, lunges, etc) when trying to build muscles. That’s why the professional athletes who do that stuff add weight training because it’s really the only way to achieve peak fitness.