Going by the prequel novel by Greg Rucka, Finn while in Stormtrooper training/service actually received regular, healthy meals to ensure health so the soldiers would be in the best of condition. So while there might be some stuff he doesn't recognise in the Resistance - maybe more snack like foods - he's actually alright.

It's mentioned in the same books that Rey swapped something for two portions, which didn't even make a day's meal. Rey doesn't have a concept of real food, and would probably end up either avoiding stuff or making herself sick.

She'll need help, so that she actually knows that this stuff is safe for people to eat.