My guess would be that your friend is confusing the our world (Where silver is, in many cultures, considered the purest material, and generally accepted to be extremely powerful when used in a paranormal setting) and FF14's world.

As best as I know, there is no 'anti-voidsent' material, just powerful metals smelted and refined into powerful aetherial items and wards. Which would make platinum the most powerful of all at the moment (Until, I suspect, Heavensward drops, and 'True Gold' is put into the game just to make the whole thing more confusing.).