If she is able and willing to do serious cardio (running, fast cycling, hard gym class, etc.), forget the tracker and do three to five sessions of 30 min. to an hour of hard cardio every week. If she won't or can't work out and needs to be reminded to get up and move around, maybe a tracker makes sense. Just my two cents and I've never used anything like a Fitbit.

I use Strava now for running and cycling. When I was in weight loss mode I tracked workouts along with what I ate in my "lose it" app and that worked extremely well for making sure I had the math right on getting the right calorie deficit.