How do you tell a 6 year old her cat died?

Oh, I feel horrible. This summer, I bought my friends' daughter a kitten. I was so excited, she was so excited. There are pictures over on my facebook page. M is a special kid. Her parents told me that it looked like Rainbow (yes, that's what she named him) had some health problems, so it's not a total surprise that he died today during his neutering. But now somebody has to tell her that her kitty is gone. She's at school right now.

Not sure what to do/how her mom should tell her.