Well, that's a slightly different question than the OP posed. There is duty-free shopping at DCA; in the middle pier, a couple of the kiosks sell "duty free" liquor and tchochkes to pax departing for Canada. Always struck me as a bit of an empty gesture.

To the OP: I haven't tried putting beverages in a ziploc, but I can't imagine any TSA staffer would notice what color your friend's mouthwash is, as long as the bottle's the right size...