Is it true that he could install the R134a kit anyway, but recharge with R12? Is the R134a o-ring kit backwards compatible? If so, he could do that to help resolve leaks. Replace the drier too. Then get it leak tested, and the compressor tested, then recharge with R12.
Regarding testing the compressor, there is some help here on how to jump the appropriate pins on your climate control plugs to get the compressor to kick on.
Middle-aged Mercedes Electrical Repairs