She looks so sweet and adorable! I'm glad she had a nice life, full with love and belly rubs.
It's completely understandable that your SO is upset but he should find peace in the fact that she didn't feel pain and passed peacefully. Besides, I'm sure he did a great job in providing her a comfortable and fulfilling life. I know that these aspects seem to not really matter when you're grieving the loss of a loved pet, but with time he should find peace and comfort.