The intentions were awesome, but he should've just said no, not use their money. Because what probably happened right after that was that they realized it was water, they said "what an ass" and went buy the vodka.
Oh please these girls were stupid enough to trust a complete stranger to buy them vodka in the first place. Who cares if the guy "robbed them". He was smart for what he did. At least two undetage girls didn't get drunk, but also didn't get in trouble in case they were caught.
He got four water bottles. He emptied two of them out, filled them with vodka, and used the water in the remaining two water bottles to fill the vodka bottle.
No mention of an empty bottle to help out with the task :-/