What can I do to help my friend? She told me that she made herself throw up and she stopped eating for two days cause her sister called her fat.

You don't have the power to control your friend, but knowledge is power.  If you both acquire some good knowledge about dangerous eating disorders (which can even be fatal), that may be all you need to avoid a dangerous brush with a very dangerous eating disorder. 

 Read everything you can find from reliable sources about eating disorders such as the information you'll find at the National Institute of Health. Be careful about the articles you read and websites you find on the Internet, because a lot of what you'll find on the Internet is erroneous and even harmful "information". There are even websites that promote eating disorders and do a great deal of harm to many young women who are told it is a good idea to throw up and starve yourself. 

 Learn all you can, tell your friend what you've learned, and tell her where to find this information. If she continues with this dangerous unhealthy behavior, ask her to see a counselor, and go with her if it helps her.

You are in a great position to be helpful, because she trusts you and has shared this very personal information with you. Keep that trust by being caring and compassionate instead of critical and scolding. Write back and let me know what happens.