Is the german "Warsteiner" a Pilsner? I've heard lately, that the german beer Warsteiner does not fullfill the german Reinheitsgebot and therefore is is not allowed to be called Pilsner, that is why they call it a Premium Verum.

Is that true and if yes, why?
I think your friend got the wrong end of a stick :p

Wikipedia lists it as a pilsner, so does BeerAdvocate and the Warsteiner site. 

I think Premium Verum is just a name.