Tell him to stop using a water pan if he is and to run the exhaust damper wide open. The dripping black stuff is creosote and water.It comes from a dirty fire that is starved for air. Closing down on the exhaust damper causes smoke to back up in the cooker choking the fire the condensation from the meat and a water pan if one is used also backs up & collects on the top of the pit making black rain instead of exiting up the stack.
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC