Parents are in a tough spot. With some coaches, it's the kiss of death for your own kid's future on the team if you speak up about any problem. I handled it by sending the team's trainer an e-mail, since the trainer handles a lot of medical stuff. My thought was that if the coaches are letting the anorexic girl run because she's good even though it could be dangerous for her health, they might not do that any longer once they know that people on the outside are aware of the situation. Well, I don't know what happened behind the scenes, but the trainer wrote me a very formal, lawyer-like dismissive response, and he was gone from the university in a few months.
The girls on the team felt awkward about going to the coach. They felt the coach would tell them to mind their own business, or say they had no proof and were gossiping, or would think they were speaking up out of spite or jealousy since she took a spot on varsity. And this girl was so darn skeletal that they knew the coach had to know, so they assumed he didn't want to do anything and would be really ticked off if they forced his hand.
This is a problem that should concern all parents of runners, because there are a lot of disincentives for a coach to intervene with a sick runner. Since distance runners compete all three seasons, sometimes parents only see them at Christmas and won't realize what's going on. From January to May your child can go from disordered to dead.