Originally Posted by Wilfred Laurier

The general geometry of a replaceable hanger seems to be designed to bend or break, even without notched or other features obviously intended to create a break. Most of them have one part about half the thickness of the average aluminum dropout, then a sudden increase up to the entire thickness of a droupout. This should (and does, as my experience has shown) create a weak point right next to where the thickness increases.

Generally speaking, they need to be strong enough, but not too strong... a blow to the derailleur should bend the hanger but not damage the frame, and not allow the derailleur to too easily snap off. If it is too weak, as I suspect the OP's friend's repaired hanger was, it can break or begin to fail under normal shifting loads. Either the derailleur falls off and the drivetrain self destructs or damages the frame, or the hanger is bent enough to cause the derailleur to throw the chain off the cassette, which, if it happens on the inside/larger cogs, will then catch your derailleur in the spokes and possibly destroy the derailleur, wheel, frame, cassette, cogs, etc.