I'm very sorry. I have premature ovarian failure, they thought at first due to being so underweight during adolescence. I didn't grow in height after age 14, and my periods never started. But they told me sometimes women recover if they're able to maintain a healthy weight later on. I didn't, but later I was found to have another, separate endocrinological disorder. That's why I wondered if there isn't something more to your daughter's condition - if Crohn's is being suggested as the cause, I can only think it's been a result of her low weight, but in that case I would have thought they would have suggested there is a chance of recovery. How old is your daughter, if you don't mind me asking? Maybe if you ask your daughter's doctors for more details, they may be able to tell you more specifically how Crohn's could be the cause, as maybe her weight is not the problem.
From my own experience, some hormone conditions can be treated very effectively in regards to the issues you mention - bone health, etc. Also, if she can manage to maintain her weight now, that will help; even if low weight is not the cause of her condition, maintaining a healthy weight now may help protect her bones. I'm very sorry you and she are having to deal with this.