Maybe the friend is jealous of the mom's life. For example, if the mom has a happy family or a successful career, the friend might feel envious and try to gain some advantage over her through blackmail.
The 'A Bad Friend' manhwa's main plot centers around the ups and downs of friendship. It follows the characters as they encounter various situations that reveal the true nature of their friendships and force them to make tough choices.
The most interesting thing was her collection of old books. She had a whole room filled with books from different eras. It was like a little library. Whenever we visited, she would show us some of the rare books and tell us stories about how she got them.