Originally Posted by autonomy

As already mentioned, modern flagship smartphones can count your steps with the built-in accelerometer. This does affect battery life.
You could get her a wearable smartwatch, e.g. Fitbit or Garmin.
If you're looking for something more sports-oriented, you can get a cadence sensor like https://www.amazon.com/Wahoo-Cycling...dp/B00L9XNFPY/ and get it streaming data to Zwift.