FWIW: You ask, does or will Zuzu know? Yes — she does and she will. When it’s time, ask if your vet will come to the house and make sure Zuzu is present; or if they won’t come to the house, take both dogs with you. Dogs too need to know when their friends and family depart. My Sam took Rose’s departure in stride. He was able to come to the blanket, sniff, and say goodbye before the vet left with her mortal remains. With another dog, in another situation, mine did not get a chance to say goodbye to her pal — not my dog — and for many years after, every time she saw a black lab, she’d run over to it with tail wagging…sniff, droop and walk away. It was worse for her than knowing.
I believe that dogs take the landmark events in life with more grace and acceptance than we humans — something we could learn from them. So listen to your boy because he will let you know when he’s done and even though we cry for them and their departure — it’s us we’re crying for. The dogs are fine.