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.
I think 'mom swap manhwa' is quite controversial and might not be suitable for all audiences. Its popularity varies depending on specific interest groups.