I'm sure your friend gets a warning his Phonenumber is already registered in the Kontalk network.
And if he continue the registration his current key will revoked or invalid.
That's correct. Because a key can for security reasons never restored from Kontalk. If you lost your key you always have to create a new one.
So he just have to finish the registration.
His old key will be invalid and the new key will sendet to his contacts as soon there contact lists will be automatically refreshed. This may take a few houres.
To get immediately the new key you can refresh your contact list manually.
And all his contacts have to accept the new key.