Unfortunately, the only option for sending one-tap-zaps on Primal for iOS is their built in wallet. Sounds like you may have set your friend up with that, though. From there, whatever lightning address he puts in his profile will be the wallet he receives zaps to. Appears you've successfully gotten that set to his Primal lightning address.

He should be good to go for zapping from within Primal on iOS. If he wants to use other Nostr clients, he will still be receiving zaps via his Primal wallet, but there is not currently a way to connect that wallet to other Nostr apps for sending zaps. Sounds like the Primal team is working on that, though.