If the warlock is at +9 to stealth (at first level?) he has put resources into this option.
He's invested a feat - stealth is not on the warlock skill list.
He's put points into Dex - dex is otherwise a dump stat for warlocks.
He's likely chosen a race that gives bonuses to stealth.
That's a lot of character resources you'd be stepping on if you denied the player this tactic.
As pointed out, the character needs to move every round to achieve this.
Any other ranged striker (as this character could as well) could simply use cover (a pillar, a corner, a region of dim light versus normal vision creatures) could use this tactic. Rangers and Rogues both have stealth on their skill lists. The warlock doesn't but can gain concealment with shadow walk.
I think (and I infer support from posts made by Mearls) that it was intended for strikers to use movement, positioning, etc., for stealth to gain combat advantage.
Also: it isn't that many rolls. Per your scenario, once per round, the player makes a stealth check. On the stat block of every creature is their perception score - right at the top.
If the creatures are intelligent or communicate, it is a free action to exchange information. So just know the top perception score at each encounter. That's the target number that the character has to beat.
Yes, it would be fair to tell the player which attacker is pointing out their position to the other attackers, and if the players disable the top perceiver, you should pick the next best.
But it should only be one more roll per turn. As I showed above, I think the player expended a good deal of resources to get this. If you disallow it, you should at least let them change. (But you'd be going against the rules as written and as intended, at least acknowledge it as a house rule.)
Also: the monsters aren't helpless.
They can get active perception rolls with a minor action.
They can ready an attack (even a charge) against the warlock when the character attacks.
They could use a power (like light, the cleric lantern spell, etc.) against the warlock.