whats a good way to express condolences for a favorite pet?

I'm sorry your mom lost her dog. Pets are dear family members to be sure. Get your mom some flowers if you can and just tell her you are sorry the dog died. I like the picture idea the first answer gave you. I lost my dog, Jake, in 2005. I still miss him. His picture is prominently displayed along with his favorite chew toy and a figurine of a little angel dog sleeping with a ball. It comforts me to see it.

Best wishes to your mom and the rest of the family that will miss your dog.

You can probably get a good picture of the dog when he/she was healthy and blow it up to a 8" by 10" and get a really nice frame, like one that says best friend or something about pets, etc. This way she can always have it, plus it will help her and you remeber the goods times.