What's wrong with getting the UUID in this function?

[NiLSPACE]

Try this instead:

local UUID = cMojangAPI:GetUUIDFromPlayerName(Split[3], true)
if (UUID == "") then
   Player:SendMessageFailure("Invalid Playername")
   return true
end

local file = io.open(friendDir .. "/friends.txt", "a")
file:write(UUID, "\n")
file:close()
Player:SendMessageSuccess("Friend added. You can check with /friend [playername]")

You're using FindAndDoWithPlayer, but that means he already came online at least once. This means his UUID is already known in the database.