At the risk of sounding like a dick, I'm going to say the money may be better spent on lessons anyway. I'm going to guess she caught an edge on her heelside and slammed backwards? In 20+ years on a board I've only had two bad tailbone falls, one was in the 1/4 pipe and I launched funny, another was when I went off-piste, and heading back onto the slope I crossed a track which launched me up and I came back down on my hip.
In regular hard charging I would say I never land on my ass. This coming from the guy who wears full upper body armor and a helmet every day on the slopes, no matter what the conditions are...
+1 but if she has a previous injury it would be good to wear something. I bought mine for rails.