T.H.
Well I think maybe you shouldn't be upset about the fact that she spent her money on her...she's right, in her eyes she was being nice. I think instead you should take the approach that although that was a nice thing to do, she needs to wary of trying buy others affections. Of course put it in 2nd grader terms but that's basically what she's doing. The one girl used her and that sucks. Discuss ways to problem solve a situation if her friend or other classmates ask her for her money again. If she wants to give it to them, she can, it's her money, she earned it.
I do not think she should ask for it back, etc. This is a lesson in friendship and "friendship" I think. In the end, I wouldn't be too upset about it, just use it as a learning opportunity!