En route Controller had an aircraft on frequency experience a lightning strike that resulted in an emergency declaration; weather affected three of the four departure gates.

Narrative

Aircraft checked on frequency deviating off the TMU restriction (1 route out the departure gate). Pilot requested 20 degree left deviation for weather avoidance; however; with head on traffic; only a 10 degree left turn could be approved until further coordination could be accomplished. At approximately FL180; pilot declared an emergency stating that that they had sustained a lightning strike and requested an IFR clearance to another airport. Clearance was given to pilot to proceed direct and maintain FL180 (with a higher altitude a possibility depending on the next sector's traffic). Pilot was asked if there was any additional assistance that ATC could provide to which pilot responded with 'negative'. Aircraft was considered an emergency (once declared by the pilot) and the flight was then treated as such. Weather closed 3 of the 4 SIDS out the east departure gate. Moderate-heavy-extreme precipitation was depicted throughout the sector; and inside our airspace. In my opinion; all aircraft should have been re-routed through departure gate unaffected by weather.