same as with Laezel if she gets killed by the gith patrol.
That's no longer the case, I think. I was on Honour Mode and encountered the Gith at level 3 - didn't want to push my luck. Lae'zel went ahead, and I figured I'd recruit Karlach in the meantime and come back with a little more power. Couldn't revive her.
Yes, Lae#zel is dead dead, if killed by the gith patrol outside of the party.
I think, it is the same with Wyll, if he isn't recruited yet.
