Platinum /gold just relates to efficiency, as there's always an efficiency loss when converting AC to DC, which is essentially all a PC PSU is, it's an AC/DC converter.
The efficiency difference between gold and platinum is actually really really small once you start doing the maths and comparing how much more or less it will affect your household electricity bill. Especially comparing a gold PSU that costs about £100 to a platinum for twice the price.
It's not 100% more efficient, probably only something like 2 or 3% more efficient, so your very very unlikely to ever recover that sunk extra purchase cost over the lifespan of the PSU.
Your mate is an idiot and he sucks at maths.
