I'm not convinced on that point. The derailleur hanger is designed to break away in the event of derailleur-in-spokes type incidents. So I see two possibilities here: (1) something in the setup caused the derailleur to go into the spokes and the derailleur hanger failed to break away as it is supposed to (I would consider this a frame manufacturing issue, assuming that Cannondale provided the hanger), or (2) the frame broke at the dropout and that's what caused the derailleur to go into the spokes. The latter case would probably be harder to defend in terms of not having been the result of wear and tear, but it seems more like the kind of thing manufacturers would cover.

I'd go back to Cannondale and argue this as a derailleur hanger failure.