I am so sorry for your friend. That has to be unimaginable pain.
I agree with others, the cards they make for pet loss are very touching or you can even write something in a blank card. I've bought a couple for friends & my parents even before I owned a dog and it meant a lot that I seemed to know what their pet meant to them. I think the biggest thing is people need to feel validated for grieving the loss of a beloved pet and feel supported in doing so.