If I have a friend join in for co-op muliplayer online splitscreen in Modern Warfare 3, and he does badly (as the second player, not the first) does his bad game effect MY stats?
No, it does not. If he is not signed in as a separate profile, he will play as 'guest' (Xbox 360). His stats will be tracked for the duration of the session as his own and upon ending the session (Guest signed out), his stats will be lost and be reset to the primary account's stats. The primary account holder's stats will only be affected by his gameplay. If the second player is signed in as a separate profile, his stats will be tracked on that profile.