if a hidden card is hidden on a turn that scuttle crab has died does the scuttle crab allow its owner to see the hidden card

Yes — if the card is Hidden later in the same turn, you can look at it.

Scuttle Crab's Deathknell has two separate effects: the opponent reveals their hand (one-time, temporary), and "you can look at their facedown cards this turn" (a standing permission for the rest of that turn). Because that second permission isn't locked to the cards that were facedown at the moment the Deathknell resolved, it extends to any new facedown cards the opponent puts on the board that turn — including a card they Hide after the trigger resolves (FAQ #10488; Rule 424.3).

Note the reverse is also true: this is just the "look at" permission. It doesn't let you play their Hidden cards, and it doesn't change the normal Hidden timing rules — the opponent still can't play that card until the next turn (Rule 811.1.b).