Is GuangZhou Polytechnic Normal University the actual name of that university? In my experience Chinese universities are either called polytechnic universities or normal universities but don't have that kind of combined name. Please make sure the name is accurate.

Working on F visas can be a problem, but I wouldn't go so far as to say it's always inadvisable to do so. In the case of the above-mentioned university, your friend has got a major problem. As in every other country, ignorance of the law is no excuse. Also, he hadn't done his homework first, and it seems he trusted his Chinese employer too much. Next time he'll know better, as people say - once bitten, twice shy!

I seem to remember you left a training center in Yinchuan of Ningxia in the dead of the night, and that you feared your name would be entered on a black list of foreigners not to be employed as teachers in China. I wonder if you have gotten that problem solved.