I've been in this situation a few times and i'll say it clear here, i'm not against people playing with their real-life friend but is it fair for 2 people who know each other well to play in a standard game without telling others?
Mainly in FOG-mode on as you don't see what they are doing,they can leave their borders lightly defended or pretend to attack each other's defenseless territories.
Because no rocket science needed, when u know someone personally u won't tend to attack him/her as you would attack others and most of the time the game ends being either of these players to be the winner.
Is it considered Secret Diplomacy?