Just had a quick look at the G40 circuits to remind myself how the relay is wired. Both terminal 86 and 87a can turn the relay on, 87a is an ignition feed and 86 controlled by the Digifant ECU.
The whole ECU system's woken up by the ignition being turned on via terminal 87a, but after the ignition goes off, Digifant can use pin 86 to hold the relay in. This means the ECU can keep itself alive for a short while after key off to write fault codes to memory etc.
The diode is in terminal 87a. Without it, when Digifant is in 'keep alive' it would back-feed the ignition circuits through the relay.