So sorry,
@Sand Dancer.
I have had to make that decision for several dogs, all beloved. The hardest part to me is that I have to make the decision for them. I can't ask them if a few more days of life would be better, despite the pain. The responsibility is all mine and I accept it gladly, just as I accept all the other downsides to pet ownership. But it's just so hard!
I'm not sure where I stand on some kind of "heaven" for pets. I do know that if I do survive death and can't be with Willow and Sophie (the most recent losses) and many others, I don't want to go, because it won't be heaven for me without them. I still feel sad for them and I extend that to your sweet friend.
All I can offer is that in my experience that sadness never goes away totally, but it never should. It does get better, that's all.