You not being the original owner doesn't change anything. Take the bird and have her get used to the environment for a few days. The cockatiel will most probably forget her original owner really fast if you take care of him. Just give her some time! Make sure that she has a very large cage, because if she doesn't like coming out a lot, make sure she has a good and spacious time in her cage. You should be ready to spend a lot of time with her, as it will be very important to create trust and bonding amongst you and the tiel.
