I have a friend whose daughter - now in her early twenties - has had very strong exercise routines, and who seeks to control her diet very tightly.
She has to do a certain amount of skipping everyday and, for example, chose to avoid a field trip that was part of her university course because she felt there would not be time or space for her exercise routine.
She did a voluntary work placement overseas and became seriously unwell because she would not/could not eat the local food. (Mostly fried on small stoves. No supermarkets full of fresh fruit and veg where she could obtain her 'special' diet.) As there were no alternatives available she became very underweight, and needed medical help - including counselling - on her return to the UK.
I would be concerned about any diet that, effectively meant, a young person was separating themselves from the great majority of family meals. Obviously we live in a society where people are very faddy and wish to assert their individuality via food choices. But most people will just say, 'Oh I don't eat mushrooms.' Or 'I want to be vegetarian' - rather than 'I won't eat any of the things that you eat either at breakfast or at lunch.'
I'd also echo the concerns about the possible long-term effects of a tightly controlled restrictive diet at a point when a young woman is starting to mature. (Did this all kick in at puberty?) Insufficient calcium can, I think, cause longer term problems with teeth and bone density.
I realise that lots of people will make affirming noises about how wonderful it is to be slim and health and sporty. To me it sounds like a degree of over-control which is problematic now, and may become still more problematic in the future.
There aren't any quick fixes but in your shoes I would be trying to access qualified advice and support.