I *believe* that as long as one raid member enters with someone with a lockout, then their gain the lock out as well. He can leave at that point, but im not sure if the origional person kills the boss if it will affect it. If you plan on doing this, I would def reccomend doing it first thing on your first raid night, just incase what I said is correct.
I think he can leave and you will be fine as long as someone remains in the instance at all times. If everyone releases to run back you will lose it, but if someone stays inside dead while the others run back it should work fine.