This plugin was made by request over on the bukkit forums.
You can find the post here: https://bukkit.org/threads/friends.495858/

Commands:

• /friend - Base command for managing your friend list.
• /msg - send a message to a friend.
• /r - respond to the latest received message.
• /boop - send a Boop! to a player.

Permissions:

• friend.max.X - assign a maximum amount of friends a player can have. By default, no max is assigned.
• friend.offline - Allows the use of the /friend offlinemode command.
• friend.msg.all - Allow the player to send messages to players who are not on their friend list.
• friend.msg - Allow the player to send messages to friends.