tablemate vs diner

tablemate and diner both are nouns.

Word	Noun	Adjective	Verb	Adverb
tablemate	Yes	No	No	No
diner	Yes	No	No	No

As nouns, diner is a hypernym of tablemate; that is, diner is a word with a broader meaning than tablemate:

tablemate (noun)	diner (noun)
someone you dine with	a passenger car where food is served in transit
	a restaurant that resembles a dining car
	a person eating a meal (especially in a restaurant)