I don't think that hit was worthy of a penalty. ...just bad luck. I hope he is okay.
I don't know the exact wording of the rule, but it's "wussified" enough that the helmet-to-helmet alone is almost certainly enough to be against the letter of the rule.
That's not really my issue with this case, however. My personal feeling is that when you have multiple defenders hitting a player from different sides, you should be held to a higher standard than "single" hits. It's dangerous as hell. (And it isn't without precedent in the rules... cut block, for example, one guy blocking low, one blocking high).