My latest playthrough I went out and scouted the Gith from above from the bridge It was pretty early in the game (2nd level I think). I did not have Lae'zel in the party yet - not even in camp. After scouting for a few minutes I decided to come back later and headed back east. Then a journal update appeared and said that Lae'zel had died. So I guess you don't even have to have the actual encounter to affect the story. I was just in the vicinity. Guess she won't be joining my party any time soon in this go-round.
