Hey gumbie! Most of those “free” apps out there love to bait you and then hit you with a paywall when you actually want the features you need—been there, got frustrated! 
The only truly free way I know is through built-in phone features like “Find My Device” for Android or “Find My iPhone” for iOS, but you’ll need access to her account. The rest usually want money after a trial. Wish I’d known that before spending hours on those sketchy sites! Let me know if you need help setting it up.