I know you don't want to hear it, but this is the problem with mixing business with friends. From the little I know of dealing with leases, etc., you do have a case against him, since he wrote you the check (or her, same thing).
But like Redz said, it really comes down to what that lost money is worth to you. Personally, I would say if you can afford it, you should just write it off as lesson learned, and hustle to get someone else in there...and never do anything like that again, without getting it in writing. Until a lease is signed (even if it is month to month), you should NEVER stop showing the apartment.
Now, if you really need the money that badly...well, you can probably go through the pain of going after it legally, which will cost you a ton of time, your friend a ton of money, and will almost certainly kill any friendship.