To put it simply, it's BS. I've heard a lot of weird misconceptons about audio but this is up ther w/ the worst of them. I'd elaborate on why he is wrong but there is nowhere to start. It'd be like trying to argue the color of the sky w/ somebody, if they don't know to begin with there's no point in going any further. What type of work does this guy do?
As far as 1 10 vs. 2 8's. I'd recommend the 10. It will give you a better overall response for the $$ compared to the 8's. If you get a 4 ohm driver (standard impedence, but others are available) then you should be able to get the most out of your amp if you bridge it into the 10 (as opposed to running a pair of speakers in stereo). This will maximize your bang for the buck.