The vast majority of original AR-10's were selective fire converted to semi (prohib), the new ones are AR-15 variants and restricted. There are indeed rumoured to be a tiny number of original AR-10's produced as sporting rifles with no automatic fire capability, I highly doubt your friend has one. I would say it's more likely that he has some other rifle, or if it is an AR-10 that is not registered as restricted or prohibited, that the rifle is not properly registered. This would be very bad indeed.