You really can't gain much strength without focusing on it, adding weights to an endurance activity like walking will likely just lead to more endurance and few gains. Ankle weights will do little compared to dedicated exercises with heavy weights focused on strength building. If that's your goal, you should be seeking out a gym and a strength building routine.
You also need to make sure you're not overtraining certain muscles with weights without also training their complementary muscles. Strong quads and weak hamstrings is pretty common in hikers and runners (good at hiking up and downhill, bad at squatting) and can lead to weird biomechanics and knee/foot problems if you don't have a well-rounded approach.
You should check out this book if you're interested in really training for bigger hikes:
Kilian Jornet recently released a book with training plans, history, and science behind training for trail running and mountaineering. I found it to have many good tips and many interesting stories.
And if you're not familiar with ultra distance training, the book will change your world
She'll want to be a good recoverer and come back stronger than ever. Get her this book, to ease in to as appropriate: https://www.amazon.com/Training-Uphill-Athlete-Mountain-Mountaineers/dp/1938340841
You might really like this book Training for the Uphill Athlete. It really does a great job covering the physiology of endurance sports. It's a followup to New Alpinism and it's more targeted for running and skiing. It talks a lot about targeting aerobic adaptions, why you need to slow down to gain those improvements, heart rate training, and a lot more.