You are young. She will learn. What might happen is that she'll be able to do that for awhile and then will have to up the calories a bit, but she might be able to keep those pounds off.
Or, she might lose some weight and then end up binging it back on.
Or..., Or... Or...
For right now, it seems that she is OK. So, leave her alone. Everyone learns what works and what doesn't work. While I agree it's not the way I would go about it. I did something similarly when I was 20.
Not all bad choices are weird choices are anorexia.