Nessa will try to lead the party into an ambush with Yurgir. If the party chooses (or pretends) to help Yurgir, Nessa can later be spoken to using Speak with Animals.
Investigating the spider carcass ( DC 14 Investigationcheck, then Arcana DC 16) in the room adjacent to Yurgir's reveals that Nessa has been charmed into being Yurgir's pet with succubus spittle. Using this information, she can be turned hostile against Yurgir, and his legionnaires (Deception or Persuasion DC 16). Succeeding an additional check (Persuasion DC 30) will subdue her and turn her into an ally if you attack the fiends later on.