Hills... New Derailleur... Broken derailleur + hanger + rear-end & damaged SEAT stay.
I think we're focusing too much on the re-straightened hanger.
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DOS, what is the condition of the wheel? Was the derailleur tangled up in the spokes?
I still don't think the derailleur hanger spontaneously fell apart. Rather, something else happened that tore it up.
I mentioned chain suck earlier, but thinking about it more, I bet, for some reason the bike shifted into the spokes. Was there a spoke protector?
This still could well be a bike shop problem, not for touching up the derailleur hanger, but quite possibly not getting the rear derailleur adjusted properly (unless your buddy tinkered with the adjustment or dropped the bike on the derailleur).
The chain was sized for the bike to shift into the Big/Big ring combo, right?