請問填寫機密推薦書的人應該跟申請報讀課程的人有什麼關係才可填寫?

Unless otherwise stated, no special relationship is required between the referee and the applicant, as long as the referee knows the applicant well enough to comment on different aspects, potentials and abilities of the applicant.

In fact, it would be more beneficial if the referee is not one having family relationship, e.g. applicant's superior at work or in other context.