"I honestly have NO idea how one would acquire a suitable SIM card for China before arriving in China." - Depending on her phone, consider getting an eSIM. This is what I now do when I travel. You order online and instal it ahead of time. It is activated once you enter the country.
This is a popular one: https://www.airalo.com/china-esim
You also have more traditional suppliers like Orange: https://travel.orange.com/en/buy-a-s...ers/china/esim
I think that strip also sells eSIMs.