"good reasons" what are they?

Do they understand that they ahve a problem, the first thing to get betetr is to realise you actualyl have a problem. You need to show them what annorexia can do and get this image out of their head of them being stick thin, you also need to be understanding though. Don't be forceful about making them stop or else they might be even more determined. Ask them why they do it what their reasons are for doing it, if it makes them feel better, and once you know all the ins and outs you can help them get through it.