What should you do if I think someone you know has bulimia?

See below. This is similar to suspecting that someone has a medical problem - they do have a right to their privacy just as you have a right to yours. The specifics of your personal relationship with this other person will determine how your actions will be received. Also, one needs to factor in that for an adolescent the parents are responsible, while an adult makes their own decisions.
Depends. Depends on how close are you to this person and what is the reason for your suspicion. Ask them if they are feeling ok or if they are having problems that you can help them with. If they let you in emotionally, then maybe you can discuss your concerns.
Be supportive. Educate yourself about bulimia; it's a way of coping with emotions and conflicts with food. Bring up your concerns, ie, "I notice that you're struggling with food. Tell me how I can help." If the person denies or minimizes the behavior, offer support and strongly encourage the person to seek treatment. Providing compassion and understanding (and not talking about appearance) can help.