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.
 

1st.) in game - go to his base on the map and try the friend request shortcut button.

(make sure YOU arnt already at 500 friends yourself)

2ns.) if that dont show up, you can go to your friends manually, in the plarium play app main screen, click friends, add his "plarium id" like Daria stated above,

or 3rd.) switch it around if THEY can try these options successfully. 1 of you are probably already maxed and didnt know it, but if you were, or they were, it should send a warning prompt informing you of the issue?