Ghost Panther has the following trait:

PREDATOR OF ZARATHOS: Poison, but deals penetrating damage. // When Ghost Panther attacks, modify his attack +1 for each target that has been damaged this turn.


Does "each target that has been damaged this turn" refer to targets of his attack or targets of any friendly attack that have been damaged this turn?

The // separates the two sentences "more" than the period.

All the power checks for is how many characters that GP is attacking this turn have been damaged.

If you poison 2 characters, then GP will get a +1 if he attacks one of those targets or +2 if he targets both of those characters.

If a friend of his damages a character and then GP attacks that same character, he will get the +1 to attack the character he is targeting that was already damaged this turn.

I hope that helps.