Hmm… So by “friendly” destroyer/s the rule book is referring to only those of the actual attacking power?
In the case of the attacker, yes. A friendly destroyer must be “in the battle” in order to affect subs. Since allied units don’t attack together, the only friendly destroyers in the battle will belong to the attacking power.
The defender, however, will benefit from destroyers belonging to an ally, since all units in the sea zone defend together in a battle.