At 30000 its likely you'll just need a refrigerant service.
Reostats are another name for variable resistors and they are just an electrical component and dont leak. Theres a low pressure cut out switch on the refrigerant line just protuding from the centre of the bulkhead in the engine bay. This switch is called the Pressostat by Volvo. It makes the compressor switch on and off in normal operation of the ac. Could he mean that? I've not known one to leak but they can go bad. You could bypass this switch with a small jumper wire on the connector (on the wiring loom side) to see if your compressor clicks in and starts up. Do this only for a short test because if you are low on refigerant, your compressor will get worn for a lack of oil. If it does start up then you are either low on refrigerant or the pressostat is bad.