Sorry about your MIL. I wish I could say no, that this is something most people just shrug off as a minor thing. I am not saying your MIL is doomed, but she is in the right place in the ICU, and here is why:
There are new, and very potent strains of C. diff out there, which I am sure you are aware (mostly bred in hospitals due to the constant exposure to antibiotics). The toxins they form are extremely damaging, and the bacteria can be resistant to even Vancomycin.
She is in a high risk group by being elderly AND weakened by chemo and cancer. So, maybe she is lucky and this is not a vanco resistant strain and they can beat this back. But, it may mean she might have to delay or stop chemo.
I don't know what sort of cancer she has or what shape she was in prior to her diagnosis, but I imagine it will matter. Presumably, prior to her beginning chemo, she had discussed her advance directive with her family and physician, so that everyone knows what she would want, should things go badly in a hurry. This is the time to "circle the wagons" and be sure everyone is on the same page. Communication is key...
PM me if you want to talk more. Things went sideways quickly with my dad after beginning chemo treatment, but contingencies had been discussed in advance. He was 75 and reasonably healthy until the cancer hit.
Jan