Is the character active and present and earning experience and treasure when the player is not present?
Yes? They are fair game. Kill them if you would kill a present PC in the same circumstance.
No? They disappear off screen for the duration into the same invincible narrative pocket dimension familiars disappear into.
I definitely prefer to have PC absent (& unharmed) when player absent. But many players ask for someone else to play their PC. In that case they're just as legitimate a target as any other PC.