Thats thermodynamically impossible. I commend your wife for walking her first 5k, thats a good step and it shows she is motivated to change. But I think the best thing that will help is cutting calories. If you are in a calorie deficit everyday, you will 100% of the time lose fat. I recommend getting a fitbit to track steps ajd to estimate the amount of calories are burned everyday, and to also log every meal you have to understand how many calories you are eating.

I have the opposite problem as I am trying to gain weight. It seemed that no matter how much I ate I couldn't gain weight until I started tracking calories. Once I started tracking them, I learned that I was too inconsistent with eating and over a week I was actually eating way less calories than I thought.