Do folks just get an electric clipper and shave that part off? She is
not going to sit still for this I can reasonably assume.

I personally think you should have the groomer do it. You can just ask them to
shave her butt area only, no bath or blowdry. In the future always keep her
butt fur trimmed short to make cleanup easier for her.

If she still has diarrhea, you need to treat that. You need to find a food that
won't give her diarrhea. With my cat I baked a chicken and mixed that 50/50
with enriched grain wild rice in cut up chunks. I gave him only that to eat and
katz'n'flodder (sp?). If I gave him anything else, he got diarrhea.