God, how I hate people like that!! Obviously she doesn't only have no respect for your cats and your wishes, she also has no respect for her boyfriend's concerns when you say he's feeling bad about it and she doesn't care.

You've got some good advice here already, and I thought of another thing that might work, some kind of shock therapy. Try to get some pictures of animals that have been hit by cars (I'm sure there are some on the net) and show it to her, and also show her comments of people who lost their pets that way and confront her with their grief. Maybe that will help her realize what she's doing.

Kirsten