I'd get her a seltzer water just to be spiteful.
clearly she's used to dealing with guys who are total chodes (the kind who would buy her a drink and sit there silently until she "goes to the ladies room")
using a compliance test of your own like t-mal suggested really is the way to go if you want to be a gentleman.
my favorite way to deal with woman who want me to buy them drinks is to make girls earn them by doing stupid embarrassing things.
I tell you what, i'll buy you a drink if you can do a handstand (especialy good if she's wearing a dress or skirt lol)
other test I like to use are
if she'll punch one of my friends
if she'll kiss her friend
if she'll grind on one of my friends
if she'll lick an ash tray (especially good if she's really trashed already)
if she'll wear the little umbrella thingy from the drink in her hair the rest of the night.
if she'll buy me a drink first.
if she'll go hit on one of my friends
using compliance tests usually defeats her compliance tests.