My German friend tells me that no one in Germany ever says "das Fischsandwich". The correct term is "das Fischbrötchen".
I'm in NRW and people do and will say Fischsandwich, it's just that nobody eats them. I haven't ever seen one single German eating an actual sandwich. They mostly just eat rolls and pastries. Which is something to say about the lesson. The lesson itself should say "Fischbrötchen" because you're eleventy billion percent more likely to be ordering ein Fischbrötchen and not ein Fischsandwich.
I'm from the south-western part of Germany. But I don't think there are many regional variations concerning the use of 'Sandwich'. However, there are strong regional variations with respect to the words we use for bread rolls. Perhaps your friend had a special context (or a special type of sandwich) in mind.