It's the rapid weight loss that is the concern, it was listed on our 'referrals' urgent criteria, so not just overall weight and that's how my dd first came into ED services. Unfortunately once the an behaviours have taken control, she then did become very underweight, so the idea they will just engage in this extreme behaviour and then back magically out of it once they hit the right BMI is wrong, absolutely wrong.
We have been on a different journey than most here, it started later (age 16) and we are now in adult services, although my dd is definitely recovering. We did not do the textbook things, in fact, things like taking control of my dd's eating pushed her to more extreme behaviours, and neither myself or the hospital admissions had much effect on that, and ultimately it was pushing her to unsafe behaviours that were worse than allowing her some control. It took the ED clinic a long time to realise she didn't have classic AN at all and in fact was prepared to do a huge variety of things to meet her goal, some of which were dangerous and extreme. I'm happy now she has some control over her own intake and is motivated to keep well to keep in college/socialising because now she's over 18, our options in terms of restricting her in where she goes, what she does and what she eats are basically impossible, she can live outside the home, and for adults the criteria for intervention with ED is much much higher (because adults are deemed to have consent, even to starve themselves to death, as a recent case has shown), so I appreciate the team pushing forward with intervention when she was younger as that's when its possible.
I think it is an ongoing issue for us, although she appears on the surface to eat quite freely sometimes, and not at others, but bulimia is not the same as anorexia and requires quite a different approach, which I don't know if the NHS services or the family therapy refeeding programmes are set up for at all, in fact, in our case, they worsened the situation before we found a way through (or rather she did, I'm not sure we did much rather than be very very supportive of her as a person).