I had this happen in my family, three years ago, when my DGD1 decided to go vegetarian. That was no problem - and DD accepted and catered for her with no problem at all. DD had done exactly the same thing at a similar age and I still had lots of vegetarian recipe books from that time, which I passed on to her.
Then, after a year of being vegetarian, DGD decided to go vegan, which did cause some concerns for DD and at first she was reluctant to go along with it but did so in the end.
It is now just part of all our lives . DGD has kept to her chosen dietary regime with no fuss and just accepts that we do not follow that regime but do accommodate her choices when catering. It works fine and she is a fit and healthy 17 year old now, at sixth form college, learning to drive and bringing joy into all our lives.
Good Luck OP - it will either fizzle out as a "fad" or will become the way she has chosen to live her life.