A mental health unit is not the right place for someone like this young lady. Unfortunately, the right facility for her - and those like her - just do not exist. This is how some people with autism end up spending years living in mental health inpatient units.
There are enough people who would benefit from a properly therapeutic environment that it must be resource and cost effective. A teenager who needs support with ARFID needs a different treatment plan to a teenager with a psychotic illness, for example.