Student or Learner
“The boy I once sat watch over as he fought off a fever is long gone, I think.”
(C. Paolini; Inheritance)
Is this some kind of a set expression?
It could be taken as such. One can 'sit watch', just as one can 'stand 'guard', though I feel that it's not common. Most of us would probably say , "The boy I once watched over...'
ps. there are three examples of 'sit watch' with that meaning in COCA.