According to this he should be at E5 at the top of the waterfall. He is quite hard to find the first time, but he really is at the top of the waterfall. Roam around the area and try to get there yourself. If you give up or you just want to get this quest done, then open the spoiler:
I remember doing that quest with friends years ago, a dear old friend, Tubadave took me there to surprise us with this quest! I am not sure and I would be interested to find out if it is still available to do! Can you let me know?