My sincere and deepest sympathy for your loss. I know your pain.
My wife and I have four dogs now, and when we met she had a golden retriever, Madison. Madison was amazing, when my now wife moved in, Madison adopted me, adn I still think of her to this day. We had to put her down due to cancer, and while its the hardest thing in the world to do, please know it was the right thing for your friend.
You gave her a good life, and she will live happier and healthier days in your memories.
Cparker