How would you bend this frame back?

Well it can't only just be bent down. It's moved toward the left (driver side) too. That means the left side of the bow is tweaked too. It would be best to analyze the assembly and determine exactly what is out of place and how to correct it.

I think I'd rent or borrow a porta-power and anchor it at the junction of the left floor and inner quarter panel wall. The business end could then be placed at the elbow of the bow opposite of the force that caused the damage. Hydraulic power could easily bend it back. (Or rip it off the car)

Or maybe take it around to a few friendly body shops and see if one will adjust it for you. I'd hope they would only charge you an hour's labor.