Just to help clear things up, it's from this findable Book in the Druid's Grove that was in the game, atleast from the first EA version.
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=921669#Post921669
So even negating whatever Act 2 datamined content there was for it, it seems it was initially planned to have Halsin kill Isobel.

Thinking about the change tho, it is abit strange that they decided to revive Isobel, given that she doesn't really do much in the game that couldn't have been another Selunite.