A friend of mine was in a big group ride and got hit from behind by another rider. The front wheel of the other bike broke the RD hanger and half of the dropout right off. I don't know anything about aluminum bikes, but I told her she probably needs to replace the frame. Questions- does she? And about how much is the replacement on that frame?
So what has actually broken? a picture would help here
The RD hanger on most modern carbon and aluminum (which the Defy2 is) are replacable, so that if damaged, they take out a cheap replacable part, not the whole frame. If much more than the RD hanger was to break, your talking about damage to the seat stays, which woud result in a write off frame.