I think that since CDG is a full-round action the target should pretty much be helpless for the full round as the attacker prepares and executes the strike. The shavee isn't helpless, just trusting. Compare to if you're exchanging a hug with someone you trust and they take that opportunity to stick a dagger into a vital organ? Would you call that a CDG just because the person totally trusts the attacker and is in a compromising position (have this occur in bed with the attacker if you think the victim has too much of an advantage standing up)?
I would give a Sense Motive check earlier on, and just before the attack, a Spot check for surprise just as normal. The Spot check represents noticing that combat is imminent, just as when you are parleying and someone decides to attack all of the sudden. Cinematically speaking, the Spot check represents whether the shavee happens to notice in the mirror (or, for extra flavor, in a reflection on the flat of the razor) an expression on the barber's face that tells him something's up. The check can be made with some massive penalty representing trust, but then that depends, too, right? Maybe the shavee is kind of paranoid, and while he is willing to get the shave, he is alert and feeling vulnerable (as many would), ready to pull back or do something as soon as that blade starts cutting the wrong way. Anyway, failed Spot means a surprise attack, and then there's initiative. Plenty of opportunity for the properly-trained "barber" to do something nasty.