Hello: I am so sorry to hear about your mother-in-law's situation. My husband has bc and although we haven't reached the surgical treatment yet, I have had a few ideas about what might be happening. Just to clarify, has anyone been going with her to the doctor appointments? It's possible that the doctor communicated about the hysterectomy and she missed that information. I say this because I go to every appointment with my husband, and we have both managed not to remember or understand all that was told to us. Living with this is a trauma and it's human nature to experience "brain freeze." I don't hesitate to ask to have things repeated and I take notes.
Given the possibility that she was not adequately informed, can she change medical care providers? This is the time for someone in the family or a close friend to act as her advocate and move on this, with her permission of course.
Someone in your family needs to start researching bladder cancer. I conducted a review of the research, web sites, diagnosis practices, treatment options, and everything else I could find when we got the diagnosis. I wasn't surprised that your mother-in-law had a hysterectomy since I have read over that detail several times when searching for information related to men and the specific type of bc that we are facing.
About the diarrhea, call the doctor's office daily and even more often until it is resolved; be sure they understand when she had the surgery, how long she has had the diarrhea, what you tried; repeat the whole story every time. Ask for other options, ask if this is to be expected, and what to do now to avoid dehydration, etc.
I really believe that my husband would have fallen through the cracks if we had not followed up, asked questions when things didn't make sense, asked for information and help on cathetors, prescriptions, procedures, hygiene, and a million other details. I urge your family to become more involved and wish you and her the best.