While traveling in Britain, I stayed at a bed-and-breakfast which was owned by a small dog (you know that’s how they think). But this little mutt was listless because 5 days prior he had been right beside his much larger dog buddy when the larger dog was hit and killed by a car. The dog’s human pets said that they had been unable to rally his spirits. He performed his meet-and-greet functions with disinterest. I showered him with affection but the most he could do was place his jaw in my outstretched hand and sigh while relaxing his weight. He seemed to ‘need’ to communicate with people (which used to cheer him up?) or perhaps by then he was confused and didn’t know why he felt so bad but it certainly seemed like grief. His human pets said he was ordinarily the life of the party...
If the sick dog dies the others will eventually stop looking for them but they will grieve for a period of time.