Did you try to make a Wipe cache partition ?
When you got a major update, sometimes some files got corrupted so WE recommand you to make a Wipe Cache Partition after each major update. This does’nt affect any of your files but can improve your device’s performance and resolve many problems that could happen after a major update
Samsung Galaxy S10+
Turn off the device.
Press and hold the Volume Up key and the Bixby key, then press and hold the Power key.
When the green Android logo displays, release all keys (‘Installing system update’ will show for about 30 – 60 seconds before showing the Android system recovery menu options).
Press the Volume down key several times to highlight ‘wipe cache partition’.
Press Power button to select.
Press the Volume down key until ‘yes’ is highlighted and press the Power button.
When the wipe cache partition is complete, “Reboot system now” is highlighted.
Press the Power key to restart the device.
After an android version update, I always backup my phone with smart switch & factory reset. I've never had any problems using this approach.
Even after the monthly security patch updates I boot into recovery and wipe cache.
Exactly, I think that the problem is that she hasn't been given the proper instructions from samsung support on how to boot into recovery to factory reset. It's virtually impossible to actually break a Samsung phone unless there's a hardware issue.
It sounds like her phone wasn't turned off when trying to get into recovery to factory reset.
Try the @CHMultimedia solution : when you plug your device to your computer and use Smart Switch PC, have you tried clicking on the "Plus" button on the top left and selected the Emergency Recovery function ?
Don't ask me if it work, but i beleive in CHMultimedia answer