This quest had me confused too. I don't think you can kill Phillipe and then cook. After I killed him, the intercom turned red and I couldn't talk to the head waiter anymore. You can still complete the quest (fully?), all the patrons are drugged out regardless.
I used the intercom after I killed him and the intercom wasn't red. :/
However, the only option I had was to call him to tell him the wine was ready after I spiked it with Med-X.
Maybe I need to go up to the banquet and just take all the "Strange Meat" I assume is going to be laid out for them while they are all unconscious?