Entrance is at 15.8S, 101.6W
The quest directions are a bit naff.
'I'll not last long if...if help cannot be found for me. Fortunately, I know of something which may be of aid. There are mushrooms which grow in some caves, which the Elves call nestedelais.
'Venture along the rise to the east that leads to the Winterheight. There's a cave there called the Rockbelly Pit where you'll find the mushrooms. They are said to have healing properties. Gather some and bring them to me. They may well be my only chance.
'Now don't get lost! The entrance to the Rockbelly Pit can be found in the Winterheight east of Frerin's Court.'