There is a few reasons this might be happening.
Doe she both have wifi on? The Apple Watch will transition to wifi when out of range of the iPhones bluetooth. Bluetooth range can vary quite a bit on numerous conditions but 30-50 feet and you are doing good. Performance and feature set can be unaffected if your Apple Watch is on Wi-fi.
Does she have Wi-Fi calling for other devices turned on in the iPhone settings (Settings > Phone > Add Wi-Fi Calling for Other Devices)? And in the Apple Watch settings on the iPhone (Watch app > Phone > Wi-Fi Calling)?
Do you both have the same cellular provider? Not all carriers support wifi calling from iCloud devices. You can see what features her carrier supports here,
https://support.apple.com/en-us/HT204039 . Also make sure your emergency address has been input on the iPhone (Settings > iPhone > Wi-Fi Calling > Update Emergency Address). A carrier needs that information for emergency services when calling over Wi-Fi.
Also verify she is signed into her Apple ID in the Watch app on the iPhone (Watch > General > Apple ID).