How did meg girys mother know about the ghost?

Answer 1

In the movie, she rescued the child Phantom from the gypsy fair where he was forced to perform for money. He had killed his captor, so she needed to hide him. She lived in the dorms of the Opera House, so she hid him in the bowels of the building. She believes he's known nothing else of life since then and knows that somehow he'd become a multi-talented genius, so apparently they'd shared some sort of a friendship throughout the years.