Can the Cellular Applewatch operate without an iPhone?
My suggestion: Buy a cheap refurbished iPhone, bond the watch to it, then put the phone in a drawer. The LTE Apple Watch doesn’t need the iPhone to be on or nearby if it’s a Series 4 or later. It is NOT possible to set up an Apple Watch on a number that s completely separate from its iPhone bond mate.

BTW, she can make calls from the Apple Watch without the iPhone involved, but it has to be bonded to an iPhone.

I’m betting you know somebody who has an iPhone 6/7 with a shattered screen or bad battery.