So what advice do you all have on how to deal with such a situation?
Eating disorders are not something to be taken lightly, not at its start not at any weight/gender/etc. Nobody should struggle with the lifelong lasting implications that an eating disorder can leave someone with.
My advice? Share this information with a trusted adult who can make the appropriate judgement call and decision moving forward. If it is a teammate, that coach is obligated by law to notify a professional like a school counselor of the situation because it is considered life threatening and and endangerment to his/herself. That counselor and the other school authorities will then make the appropriate phone calls, contacts, and referrals necessary to deal with the situation. Then, it gets moved to a professional and the parents who can handle the situation. Hopefully it is early on in the situation and this hasn't been an an ongoing problem; eating disorders are a result of a need to control a situation and usually aren't born out of dissatisfaction with body image. But, when you lose yourself and get a warped perception of what you're worth that one thing you can control (food) becomes the weapon of choice in dealing with all of life's stresses.
Very few make it out alone and unassisted, a team of passionate health experts can help you get a better immediate physical recovery but the psychological relationship with food is something that needs to be more deeply looked into. With that, you have to figure out ways to handle and deal with stress and control in other ways that aren't abnormal or unhealthy food restrictions/binges/purges. Remember that a healthy body doesn't just produce a healthy athlete, but it produces a happy human. You can live to see your friends, family, kids, and anyone else grow up and physically move. If your bones are destroyed and your hunger cues obliterated from abuse, how can you ever grow physically able enough to see the small beautiful things in life?
My advice besides getting a team, letting someone who can HELP know what's going on, is to let what will happen happen with regards to athletics. They can't do their other passions and hobbies either if they're immobile or dead. What's worse? Having that guilt on your hands or having the confidence to speak up to protect someone?
For the person dealing with the eating disorder. Seek help. If you won't, ignore the scale and calories. Try to stick to a routine with eating like you do your training. That control of routine can replace the control of ignoring your body's nutritional needs. Make it the same goal you had before to continue with your eating disorder to nourish your body. And lastly, force yourself to have 1 thing you wouldn't normally consume once a week. I did this in recovery and it taught me a lot. A mountain dew at a Friday night football game, a snickers bar at the gas station, a slice of pizza after dinner because friends were still hungry. This taught me that life goes on, I don't have to control it all all the time, and that I didn't magically lose my athletic ability, I didn't spontaneously become obese, and I was still the same person people could love.