A DD belonging to the same country as the aircraft is required for the aircraft to engage the sub. For example, in the above assume the CV belonged to the US and there was a British DD in the same sea zone when attacked by a German sub. The sub’s surprise attack is nullified by the DD, but the British DD cannot “guide” the US ftrs to attack the German sub. However, the British DD and the US CV can both return fire.
Sorry, but this is incorrect. When defending, a destroyer only has to be friendly to an air unit and in the battle in order to allow the air unit to hit subs. It need not belong to the same power.
When attacking, the destroyer must belong to the same power, as a destroyer in the sea zone belonging to a friendly power will not participate in the attack.