As The Smoking Gun puts it, Seay "does not seem like an amateur when it comes to partying." They found his MySpace page, which contains numerous photographs in which Seay is either carousing in a bar or has a clearly visible drink in his hand (which are helpfully pointed out with red arrows). His MySpace page has "gettin' drunk and having a good time" as one of his interests, and friends' comments include greetings like "Hi Drunk Friend!!!" and "Hey Hey Justin Seay, Drinks like a fish everyday!"
The Smoking Gun provides pages from the lawsuit and photos from his MySpace page for handy comparison.
(Hat tip to Dave Palmer on the SKEPTIC mailing list.)
UPDATE: Radley Balko makes some reasonable criticisms of the Borat movie (which I've not seen).
UPDATE: You can find more Borat backstory here. The deceit used to make this film strikes me as quite unethical, though I have little sympathy for Mr. Seay, described above.