Hey everyone.
"...it would have to be tin plated on the inside or you couldn't ever drink from it. Brass is poisionous and should never be used to store food or consumable liquids."
I think your friend is misinformed. Don't look now, but there are probably brass fittings in your plumbing.
Brass is mainly composed of copper and zinc. Some brass alloys contain lead, for example, to make them more machinable, but that's not a good material for a cup.