ZT一个

My father did it again
His AC starting blowing warm air in his 90 GT. So he went and bought R134 and added a 1/2 can to the system. He knows its R-12, but did it anyways. It blew cold air for a few days, but now when the AC is on the compressor starts to squeal. Any ideas? Did he royally screw up the AC system? What exactly is the process to convert it?
He tried to redeem himself by buying guages so he can tackle it himself. Any help or input would be appreciato!