Fredo needs Mike's help to order a Banana Daiquiri at the Havana cafe. But wouldn't a Havana club-goer(i.e Fredo) know how to order his favorite drink??
I think the whole bit was a ploy by Fredo to keep Mike from suspecting that he and Johnny were in cahoots.
(Maybe Freddie inherited a bit of his old man's 'Sicilian Cunning' after all. )
)