friend vs messmate

friend and messmate both are nouns.

Word	Noun	Adjective	Verb	Adverb
friend	Yes	No	No	No
messmate	Yes	No	No	No

As nouns, messmate is a hyponym of friend; that is, messmate is a word with a more specific, narrower meaning than friend:

friend: a person with whom you are acquainted
messmate: (nautical) an associate with whom you share meals in the same mess (as on a ship)

friend (noun)	messmate (noun)
a person you know well and regard with affection and trust	(nautical) an associate with whom you share meals in the same mess (as on a ship)
a person with whom you are acquainted
an associate who provides cooperation or assistance
a person who backs a politician or a team etc.