Apple Watch cellular

That’s not really possible. Unless the watch was not setup with her iPhone, but instead was set up using Family Setup from another person’s iPhone. In which case it make little sense, since it doubles the phone lines.

For normal usage, setting up the watch from a paired iPhone, means the watch takes on the paired iPhone’s number and does not get an independent number.

Yes, that’s the wya it works. It takes on the paired iPhone’s number for calls and messages. It does not get its own number.