Hi,
The first link on the Page I linked goes to https://support.apple.com/en-gb/HT204045
It lists:-
"Kenya1
Airtel
- FaceTime over Cellular2
- Personal Hotspot3
Safaricom
- FaceTime over Cellular2
- LTE
- Personal Hotspot3 "
The Notes for items 1, 2 and 3 are:-
"
- The carrier, not Apple, provides technical support for iPhone in this country or region.
- You need iOS 6 or later to use FaceTime over Cellular. You might notice decreased performance on some carriers while using FaceTime over Cellular on 2G networks.
- May include an additional fee or requirement. Contact the carrier for more information.
- Carrier doesn't support MMS.
- FaceTime may be unavailable if your device originates from Egypt and becomes unlocked or is used in another country or region."
If the Phone is not dropping a Video Call properly or fully then it may be "stuck" where settings cannot be changed.
However swapping the SIM card which would cause the Phone to reboot should have sorted this.
In the UK the Fibre and Copper wire to the house are supplied by British Telecom but they in turn can Lease the lines to other Providers.
So you could be paying for just a LandLine for a phone service over copper wires from the Exchange to your House.
In addition you could be paying for slower speeds of Broadband over those same wires. (250Kbps max)
You could opt for a Fibre to Cabinet service if the local exchange offers it and get speeds up to around 75Mbps download and about 10Mbps Upload.
If you are using a Mobile service here then you needs to be getting at least a 4G service to be able to Video call properly.
My iPhone connects to my home LAN as the phone coverage literally changes as I wander around the house.
In some places I get 4G but only displaying 1 bar. More often I see 3G listed.
In a few spots I get just an EDGE listed service.
Right this moment my iPhone is connected over USB to my computer and is showing no service at all.

I would check to see if she can use WiFi from a LAN network.
Even then an Internet Cafe or possibly a Hotel's free service may be being used by so many that it will not be able to have enough capacity for calls.
However being able to join such a Network may free the device up to switch to Audio Only call.
If she backs up her iPhone then she might be able to "Restore" the device back to when the App worked fully.
Ralph