The New Scientist article lists Almonds, Broccili, and Baked Beans as significant non-meat/dairy foods. Wiki on Broccoli lists %DV (Daily Value) as 4% per 100gms, Baked Beans are not that much less - but eating 3Kg of the stuff a day?
Sen tell your niece to concentrate on some of those products, not sure about Brussel Sprouts or Cauli, but being related might be almost as good alternatives.

I posed this question to a vegetarian, who said she took Vitamin B₁₂ supplements. I left her with the task of doing her own research, maybe she will. We all choose our own route to purgatory.

This is about knowledge not statistics of the likelihood of problems. If you are the one in 10,000 it is not irrelevant, though it may be ignored until.................

I well remember my dear ex-GF, vegetarian for 20 years. Until she was diagnosed with cancer (& secondaries), then the smell of bacon could no longer be resisted. She asked me to be with her when visiting the oncologist. Geeeeze if I can help just one someone it might ease that memory.