I wouldn't CDG it -- the target isn't helpless. And as has been pointed out, a CDG isn't very cinematic. The barber rolls some dice, the player rolls one, the character's dead.
The character is flatfooted, loses any armor or shield bonuses (but retains bonuses from a ring of protection or amulet of natural armor), and has a -2 AC penalty (sitting). The barber is arguably on higher ground, and could claim a +1 bonus to attack.
The character either gets a Sense Motive check or does not, DM's call. A successful Sense Motive check negates the flatfooted status, but not the lack of armor, the penalty for sitting, and the barber's "higher ground" advantage.
If unsuccessful, the barber makes his attack, including sneak attack damage, and does the whole assassin death attack thing.
If successful, roll initiative -- remember the PC is sitting and has to get up (move action; draws AoO). If the barber wants to keep the character down, it becomes a grapple.
Cheers
Nell.