Hi Bill,
I've had to put down 3 dogs, and in our family, the dogs are just that, family. Each one of these great animals were our friend, protector and companion. It is one of the hardest things I have ever done, and there is such great sadness when we have lost a dog.
Just knowing that you care is a great place to start. It's up to you what you think may be appropriate, almost any gesture of good will and remembrance of the animal works for me.
I've had people give us cards, hugs, phone calls, knowing looks; one dog even warranted a bottle of Champagne from a friend to toast his wonderful life (the same dog that's in the animated .gif on my website).
It's all good, and very much appreciated. And I am very sorry for your friend's loss, I know it all too well.
HTH,
Chris K