You will need an Apple Watch Series 4 or later to detect hard falls.
If she plans on leaving her phone behind and relying on the watch to be able to call emergency services if a fall is detected, she will need a cellular capable watch and a plan to go with it. Otherwise the watch must be connected to a known Wi-Fi network and Wi-Fi Calling needs to be enabled for emergency services to be dialed from the watch.
Refer to the links below for more information:
Use fall detection with Apple Watch - Apple Support
Use Emergency SOS on your Apple Watch - Apple Support