How do you help a close friend you believe is bulemic?

The person has had some serious life reversals over the past couple of years and has steadily lost weight and become emaciated the last 2 years. In a restaurant she always excuses herself after eating plenty of food and goes to the restroom. She would be mortified when she realizes I know there is a real problem. I fear she is in a life threatening spiral. Recently she wrote her memoirs and is only in her mid 20’s.