Why not? It would make sense for Wyll to be rewarded if Karlach is killed in camp. It's not a build making item, but it does half all incoming fire (and situationally cold) damage, while dealing 2-16 fire damage to melee attackers. Not bad for a robe you can get very early on.
However, I tried to find instructions for this on Reddit:
"Steps for the loophole
kill karlach
talk to wyll so he knows you kill karlach
keep wyll away for a bit
use scroll of revivify on karlach (don't let her see you when she wakes up)
long rest at camp
get clothes from devil lady
go back and talk to karlach
get karlach to join you
give karlach new fire clothes
raise middle finger to devil lady"
I'm not sure if your mistake was bringing Karlach's corpse to camp, or if they actually fixed it.
