The hanger should have been replaced. Never bend a bent one back into compliance. Weakens the material and under extreme load it can fail, as you witnessed. When we tell people that the hanger needs replacement they freak out and tell us to simply bend it back. We hand the bike back to them if we are unable to convince them of the reasoning behind the replacement policy.
Although there is no way for the shop to determine exactly what led up to and caused the hanger failure as pretty much anything could have happened between the time the bike left the shop and the time it broke, perhaps they can convince the supplier into a bro deal or crash replacement deal on a replacement frame.