Im almost possitive my best friend may have an eatting disorder. How should I help her?

Her new years resolution was to workout and to loose weight so she quit eatting junk food which was fine but then little by little she just about stopped eatting compleatly. Then when she does eat its like an apple or a rice cake.

The other day we shared a small meal and like 5 minutes later she was in the bathroom. I got worried so I went and listened at the door. And I could hear her puking. I said something to her about it the next day and she didn't say anything. I told how worried I am about her and she just looked at me and said "you don't have to worry I have a good head on my shoulders".

She still isn't eating much. And her weight is dropping rappidly. She's at the gym almost everyday working out and breaks plans with me to workout.

I just want to know how to help her. I know I shoud tell her mom but I don't want to worry her if I can help.

Also she's 5'2 and 127lbs.

What should I do? Help me please. If you have any questions on the situation email me.