right-hand man vs man Friday

right-hand man and man Friday both are nouns.

Word	Noun	Adjective	Verb	Adverb
right-hand man	Yes	No	No	No
man Friday	Yes	No	No	No

As nouns, right-hand man and man Friday are synonyms defined as:

Other synonyms of right-hand man include chief assistant.

right-hand man (noun)	man Friday (noun)
the most helpful assistant	the most helpful assistant