Well, if
sm_friendly_remember = 0, then all you have to do is slay the player to get them to exit friendly mode. If that cvar = 1, then one way to get them to leave Friendly Mode is by forcing them to toggle it themselves, through
Fake and Force.
As for a built-in way to target other players, stay tuned...
I'm also trying to get a timer system in place, where a player can only stay Friendly for a limited amount of time (during 1 life).