There are two ways of doing this quest and the outcome depends on which way you choose.
If you want talk to Jan about Sara she will appear at the bandit camp later on in the game. If you talk to Tristan about Sara she will appear near Severin at Tilda's farm later on in the game. After to have spoken to either Jan or Tristan go back down to the abandoned house and talk to Sara and she will give you a strong healing potion.
Philanthropist (10 points)
"Rescue Sara from the wilderness" completed