Hmmm I'd probable ask the person in question if they would like to share their own thoughts on why they speak bad of their mother, offer them a safe none judgemental space to openly discuss any issues the relationship has endured, hopefully find a solution, making steps to move forward. It takes an awful lot of energy being resentful. Hope your ok....
It's hard to speak on that because at my age I know that there are several sides to every story and only one is the truth. I do not judge someone with harsh words for their parents because their experiences shape out attitudes. I do not engage in such conversations, though, if that is what you mean.