My chronic renal cat took a turn for the worse after taking metronidazole - help?

It could be she's sensitive to metronidazole, or it could be whatever the infection was has gotten worse. A small amount of yogurt would help if she had diarrhea from the antibiotics, but not so sure it would help in her case. If she hasn't been drinking because she isn't feeling well, that will tax her system and make her feel worse. Kidney cats get dehydrated easily. She may need to be on an IV for a while to rehydrate her and flush out her system, since kidney disease also causes toxins to build up in the bloodstream. Your vet would know better than I, I don't know her whole history. And if possible, it's better to stick with one vet than bounce back and forth, so that she isn't given medications that interact badly with one another by accident.