Hi.
Sorry to say, but You're probably SOL
AFAIK.
The meals are not out of Your mouth now are they? It's Your buddy who lost the nut and the washer, he's the one who's going to buy them, right?
I have misplaced my thread gauges ATM

, but I think there was a similar thread quite a while ago. I'll try to find that thread or the gauges, but M10*0.75 is a size I seem to remember.
That
IS the same thread as in some potentiometers and other electronic hardware, so a trip to a RS or similar may help if they carry metric parts.
Edit:Found some regular M10*0.75 nuts of the same thickness with yellow passive coating and slightly larger outside dimension (14mm vs. 13mm wrench). Those fit. If all else fails, drop me Your adress and I'll send You a few.
Regards
Sam