Maybe he should get up on the roof (or have someone do it for him) and check around the skylight.
If your friend had it done, it's possible it was not done properly.
Dicor self leveling caulk or Eternabond tape does a great job of sealing around skylights.
Also, tell your friend to have the entire area near the skylight checked. It could be leaking a couple feet away and dripping into the skylight.
The whole TT roof should be inspected for leaks.
Maybe someone will have a better answer. I'm just telling my experience with a leaking skylight.
