Relays electrically isolate the control circuit (low current) from the circuit being controlled (the a/c compressor clutch, which is higher current).
My guess is that the relay was not sized correctly for the A/C compressor clutch. But I would think that a fuse would blow before the relay crapping out?
The control side of the relay could also be failing? Most bosch relays need 8 volts to energize the coil (which pulls the contacts) and the Drop-out voltage is usually around 1 to 5 volts. So maybe the coil is not sized correctly