Hello, Commander! In this case, I recommend trying to add your friend through the Plarium Play platform itself. If you have him as a friend on the Plarium Play platform, he will automatically be added to your friends in the game. 

You can try asking your friend for their Plarium ID and then searching for them in the ‘Friends’ tab. If it still doesn't work, let's contact our support team so they can help you. 

Here's the link.