Hi and thanks for this query.
I am so sorry to hear what your mother in law has been going through.
From her history, her prognosis is very very poor. If she is conscious and can talk, then it is her place to make an informed decision whether to continue with the chemotherapy
or not. Doctors will rarely want to stop treatment against the wish of the patient. Though knowing the prognosis is so poor, they are not the want to make the decision to stop or not. They just need to let you know that refusing treatment is one of the options you have. And if they cannot tell you what the exact benefits are, then you ay want to stop it.
I wish you well and hope this helps.
Wish your mom inlaw all the best out of this battle.