Maybe my wording was not clear, but indeed you cannot get the hair if killing Ethel.
Apologies, I should have made clear that my response was to the OP rather than your more recent reply, and specifically the statement ..
I negotiated with her to save her victim and let her go, and I got nothing mechanically from doing that. At least, nothing I wouldn't have gotten from also killing the hag, who was left with 12 hp.