I've not heard from him since this morning, so maybe he wiggled the connections and it hasn't happened since.
The info was that it had happened 3 times, and it stayed like it to the end of the game. I didn't clarify with him if he exited to the menu and whether down was working then. Or what he did between it not working and it working again. Have to try that if he calls back. As my ideas above indicate, I can only think of mechanical causes up to the encoder.
He is only interested in 1 game, Defender, even though I set him up with left/right buttons for Galaga he insists he's not going to play it! This does mean that there are 2 spare microswitches he can try if that does turn out to be the most likely cause.
I was in two minds whether to spade or solder this, and still am for the next one. I chose spades mainly because the earth daisy chain looks neat and is easier than soldered solutions. Solder would mean I wouldn't be worried about the connections coming loose in transit. But solder would also make it hard for him to swap out a microswitch, which wasn't something I'd even considered before.