1992 Ford F-250 134A in an R12 system

Air Conditioning problem
1992 Ford F250

I took my pick-up to a mechanic to get air system recharged, the mechanic put 134a in my r12 system. I did not realize until a few days later when the air would no longer cool, I took it to another mechanic and he said because of nothing being converted and the r12 still in the system I was going to have to replace my whole system now, is this true and if so should the first mechanic be responsible for the damages?