In my view, just any general purpose rated power diode will do.
But I think the focus is on the Relay! Rated at 7A for only 6000 cycles. The Relay that your friend said is warm, replace it!
Measure all the relays contact resistance.
Power up the relay to close & Open, [apply external power to A1 and A2 Pins]
Apply by clip on 18v, relay should close [put audible], reduce voltage to 10v, relay should close, slightly knock the relay, it remain closed. Repeat few times. Any failures, replace Relay.
then measure the closed contact resistance [Pin 11 and Pin 14]. Change or clean the contacts if any abnormal resistance is observed. Replace the Relay is the best as cycle life may have been exceeded.
Also measure the coil resistance A1 and A2, should be between 1100 ohms to 1400 ohms... Deviation out of this window, change the relay.