@Marcus *********
send 'em to the nearest immigration office to where they stay and have them ask about any "emergency extension" based on the corona virus.
Failing to get that they could get a 7 day application for extension denied stamp for 1900baht.
OR
As people said, fly 'em out and back (they don't hafta fly to china and back just out and back).
Most chinese know they only get 15 day visa on arrival and buy a real tourist visa from a thai consulate in china before they wing their way here if they need to stay longer.
Good Luck