First & foremost sorry for your loss.

We went through something similar (healthy dog got cancer and had to be put down at 8 years old) and while it's too late to help you I feel the need to post this because for us I feel it made all the difference. Our other dog was present when she passed and was laying next to her in bed (see sig for details on in-home option) and as soon as she went he got up, sniffed her, and hopped out of bed and walked away. Things did change somewhat but we feel he handled it very well considering they were by each other's side 24/7 from when they were puppies. We had lots of horror stories of friends taking one of their dogs to the vet and coming home to confused dogs who later would howl in the basement at all hours and other heart crushing stories. While I have no way of knowing if this was the difference or not or if it would have been different in our situation or not I firmly believe he knew when she passed as evidenced by him laying by her side up until the exact moment she was gone and I also believe that helped him cope with her being gone.

Again, I'm sorry for your loss and I know this doesn't help you much but it could help someone else.