Looks like a wet ramp. He appears to be facing the bright lights of the terminal. Reflective water hides the yellow line and taxiway markers look the same from every direction so if 4 are lined up on two adjacent taxiways then it looks like another taxiway that isn't really there. I've taken a turn like this before except I was slow enough to see the grass when it appeared from side scatter in my taxi lights and corrected in time to find the actual turn-off 100' further. This is actually a really easy mistake to make. Since then I've had this recurring dream where I run off a taxiway on a dark and stormy night and then sit around smiling at the passengers after we deplane in the mud and they try to keep dry holding the seatback magazines over their heads. What a night!