I use fitbit, and it's not about the calories at all for me. I know some use it for that but everyone I know who has one, including those on weight loss journeys don't use the calorie feature.
Its about the step tracking (we as a species are getting more sedentary) which motivates me to move abit more, tracking my fitness sessions and how well it captures that. The ecosystem fitbit provides is a lot better than others I have used as in how it displays things . It also has menstrual tracking, which depending on how old she is may be really good.
I really enjoy the challenges you can do too, like walking distances of real life hikes. It's quite motivating.
I find they are pretty well made as well and very simple to use.