How difficult is it to transfer to a different field of study in a master's program compared to getting admitted in the first place? My friend in China has been admitted to some schools in the US, but hasn’t started his program yet, and is having some second thoughts about his field of study. It would be in the same school, from a different engineering field to CS.

Would it be easier to transfer once he starts or just re-apply with a different major for the next cycle?
This depends on both the institution and on how different the two fields are. If two fields are closely related they could conceivably share an admissions process and the department might not care much which program a given student follows. But in other situations, it might be quite difficult. 

A change from engineering (other than, perhaps, Electrical Engineering) to CS is a pretty big jump and the two programs probably have different entrance requirements. There might even be a waiting list in one or both programs. 

You can ask the institution for guidance, of course, but don't assume it would be easy. Asking would be better than just delaying.