You need to download/install the "Find My Phone" app, from the App store, on your phone or iPad, then login to Find My Phone, using your Mom's Apple ID. If online & Find My Phone turned on, on her phone, you should be able to locate it.
The Find iPhone app is for finding a phone, not for making it findable. Having it installed on the stolen phone is of no use. The Find feature has to be enabled in the iPhones settings for the app to be able to locate it running in another iOS device