Earlier quoted context omitted.
I'm pretty sure daily walks (no heart rate elevation, not HIIT, no muscle being built, etc.) didn't contribute significantly to your mother's weight loss. Moving around is not a significant contributor to weight loss[1]. Diet is the most significant contributing factor to gaining or losing weight. Moving in with someone new, new environment, new dietary changes, etc. might've done significantly more. 1. https://www.n…
Weight loss is simply calories in - calories out. If she walked an hour a day and ate the same amount, she was burning around ~300 extra calories a day. Pretty sure her daily walks did contribute significantly to her weight loss.
If she does more activity, she gets hungrier. This will pressure more eating. Fighting that is a losing battle for majority of dieters.
Furthermore, consider that the body is a lot more complicated than "calories in, calories out". It can change your hunger level, lower the body temperature, reduce energy spent on fighting disease...