In their diagnosis, did they bother taking note of what the pressures did (if anything) when the compressor clutch was switched on and confirmed engaged?
It's not unusual for these old Sanden SD7V16s to have the internal, mechanical refrigerant control valve stick, causing the swash plate to not change angles to make for effective displacement of the refrigerant. Can't move refrigerant well, it doesn't make much if any pressure differential. Having the pressure differential is what makes it cold.
When the compressor is off, the pressures on the high and low side equalize. When the compressor is on and working correctly, the low side should dip to 25-30 PSI and consistently stay there no matter the ambient temperature. The high side can vary greatly to somewhere a little over 100 PSI to 250 PSI depending on ambient temperature.
But if, say, you have 90 PSI (give or take a fair amount depending on ambient temperature and charge level) on both sides and when you switch the compressor on, the pressures don't change or change very minimally... most likely is a bad RCV inside the compressor. Most shops are only going to want to replace the whole compressor since the vast majority of the wholesalers out there don't have internal compressor components.