Income tax question?

My friend was common law married and he has a daughter. They split up and his wife moved out of his apartment last September. His wife and kid moved in with his mom. She doesn’t work and he gives her $200 a week. For taxes he claimed his daughter which made his ex and her mom angry since his ex’s mom wanted to claim the kid and his ex. Ex’s mom hasn’t filed yet, but he already did. Ex’s mom wants a copy of his taxes so she can get it ‘notarized’. I can’t imagine why she would want that and what she could do with it. My question: was he right to claim his daughter and can ex’s mom do anything about it? Thanks in advance guys!