Is it normal to for a girl to be able to cry at a whim?

I've been going back recently thinking about my good friend's girlfriend in high school. She cheated on him at least once with most of our friends and was very manipulative and conniving, her brother was the same way. She also flirted with me a bit and would ask to hang out. One time I remember we were at the mall and she picked something up and told me about how much she wanted it, then she started crying about how poor she was and like the naive teenager I was, i bought it for her. Another time we were hanging out with her, my friend who was dating her, her brother, and a couple of other friends. We were in a music store when they decided to pull a prank on me by having her make herself cry while she was standing right next to me with her brother shouting "Hey man! Why did you make her cry?!" It was pretty funny at the moment, but in retrospect I find it bit strange. Are girls able to do this pretty easily or was she a bit of a sociopath?