Its a stupid way to catch a school shooting. Especially because its in the news now, potential future shooters know not to post anything or message about anything. I'm not arguing that point, but we can't talk about real ways to stop mass shootings so we are stuck with this conversation instead I guess.
I think being in private isn't an excuse to say anything you feel like. There should be social consequences to what was said, but it should likely just be her friend giving push back to the joke even just as distasteful.