Is some protection necessary for a RS232 link between PIC-MAX232-Computer?
I don't know if I understood you correctly. I envision your circuit like this:







simulate this circuit &ndash; Schematic created using CircuitLab

I would couple earth with my device ground over a high ohmic resistor. But I do not know if this is mandantory. I believe your problems are more likely to result of some problem with electronic discharge or overvoltage on your signal wires. 

The secure option for devices out in the fields and in higher production quantities would be having an optical isolation. If that is not possible/too expensive, try to protect the interface with some ESD and overvoltage protection devices. There is (as far as I know) no standard approach, because every environment is different. What works in a lab environment may fail out in the field. 

Most of the time I see varistors for overvoltage, coupled with some ESD protection ICs. Serial resistors with values from 100 to 500 Ohms reduce burst. Additionally you will have to pay close attention to your PCB routing, as the deflected ESD energy has to go somewhere (ideally directly to GND and off your device, not trough your microcontroller). If the client uses signals with too high voltage, limit them with a zener and a resistor in serial. This also makes error searching easier, as a burnt zener is a foolproof sign for misuse by the customer.

Side note: depending on how your connector looks, some clients will destroy the interface by plugging in wrong cables, wrong polarizations...