It's so hard to know what to say in these tough situations. You feel so bad for them. I would just let her know how bad you feel for her and there your there anytime she needs to cry or talk or just be with someone. I'm guessing this just happened recently so it's very fresh for her. There are some really nice cards out there for pet loss so maybe you can send her a card. Here's a link with some nice gift ideas to give to someone who just has lost their pet maybe something like that would help her too. Just to know that there's someone out there that doesn't minimize the loss because it's a pet means alot to the one grieving.
Pet Loss Bereavement Gifts