Ehm usually when a friend messages you they are in a different lobby and are unable to see you.
Therefore I don't really see the point in this suggestion :x
Actually, something could be done similar to how messaging works with the essentials plugin that many servers use (CubeCraft doesn't have it, I believe).
Basically, if a player is afk, and they are messaged, the message will be sent, but in the sender's chat, a colored message will appear saying "Player [playername] is currently afk and may not respond".
You can also have an [AFK] tag appear infront of player's names in /f list. Also, like in essentials, you should be automatically put into /afk when you haven't moved for a certain amount of time.