Where my seat sits, I'm pretty certain my angle would be less then 45 degrees. Once I get the cage in mine will be mounted to the bar that will run on the uprights between the bpillar.
If he just wants it mounted for now for track days it'd be worth doing.
As for a big crash, if you're done up properly the angle shouldn't effect any impact on body I would think as there's no slack etc that gets taken up...