What should I say in a card to someone who's dog just died?

My mom's coworker's dog died. However, the emotional situation is more complicated. She got the dog last September, and it was an adorable little puppy and she loved the dog so much. She also spent a ton of money and time on it. She payed $2,000 for the puppy. When she left a dog at a day care for a week when she went to vacation, she came back, and the dog was sluggish and slow. They took the dog to a hospital and they revealed many problems about it. The dog ended up dying, after 8 months of life. I kind of feel obligated to right her a card, but I don't know what to right. Keep in mind that she has a ton of emotional stress, and a "sorry" wouldn't really do. So what should I put in the card?