why is it we spend so much time trying to avoid hurting someones feelings, or getting them upset? When they are not returning the favor? No, means No. You do not owe her an explanation nor do you need her to agree with you.
It is not confrontation, It is simply not allowing yourself to be a doormat.
You did not buy it for her.
I have told my sister, My sister! I am not paying full price for you to have it for free! With my sister, I say, pay half and we share, but for what your talking about No. She can go buy it brand new.