Think about what the Mother's message to her daughter is by doing this: I wish you didn't exist. And of course, the daughter will go along with it to win the Mother's approval.
If the daughter's capacity to think is not affected by her mental impairment, I would say, yes, she would probably want to comply with her mother's wishes that dying is preferable than living under her circumstances. Whether the mother has influenced her daughter to the point of abject worthlessness, and the daughter has filed this lawsuit -which I seriously doubt- or, whether the mother filed it under her daughter's name -which I believe is the case- I find it totally reprehensible and absolutely heartless.