Welcome to Pet Talk. I'm sorry to hear of the passing of one of your kitties. You're probably right; the surviving kitty more than likely is depressed b/c of it. However, I concur w/the above opinions; take your kitty to your vet and have her checked out to be sure it isn't something physical that's causing her lapse in using the litter box. Give her a gentle pet for me and please keep us posted.