Get involved now! If you don't get involved now, it may be too late for her. Get to her while she's still, hopefully, fairly healthy.
Many bulimics and anorexics are preoccupied with cooking and preparing, talking about dieting, and anything related to food. Casually mention that you're trying to drop a few pounds a see what she says.

Get involved now, I can't stress that enough. An earlier intervention can mean the difference between life and death. She needs this, whether or not she sees it just yet can be...heartwrenching. The earlier this is done, the easier it may be to help her because she might not be so set in her ways.
Get help for her now!