I don't doubt that chimps grieve, our own pets grieve, some more than others. They become very unhappy for a while after a person they live with or a fellow pet they get on with, dies. I've had this experience twice in my own pet family. In one case it was extremely traumatic. It didn't last of course but while it did it was distressing for us and my cat for about three weeks. So it doesn't surprise me that chimps grieve, as do penguins, cows and elephants. Probably many others but I don't know enough about them.
I can't see why either Hope or Sassy would object to the idea that animals grieve. I know they say we are not animals and all that but that doesn't mean they don't recognise other animals have feelings, they've probably had pets. I'm sure Sassy has, she'll correct me if I'm wrong.