Maisie, I'm guessing it might have been one of our guys that told you that - I know the marshal it happened to! Having said that, these types of stories do tend to wind up in the Marshal version of "urban legends"
I was actually going to mention that we have had a couple of drivers with prosthetic hands/arms in Ireland - and we also have a very competitive karter with a prosthetic leg.
If your son's girlfriend is actively interested in competing Hans, get her to have a look at this site -
http://www.justmobility.co.uk/bmsad/ - it's aimed at UK residents but I'm sure they would be able to put her in contact with people in Canada that can help.