This is structural component. It cannot be bent out, because it is aluminum. Any cosmetic fix is just hiding a potentially big collapse of the frame.
Here's a simple experiment to show the strengths and weaknesses of aluminum as structure:
1. Empty a beer can by any method. Don't dent it at all.
2. stand on the can's top -it will support >250lbs
3. While someone is doing this, make a small dent in the side of the can, it will collapse in milliseconds.
Aluminum frames cannot be bent back safely, and the cannot be dented with compromising integrity. This is not cosmetic damage.
To the OP: the original offer of $800 from the seller to buy another frame was generous. This bike needs a new frame. There is a very good reason why a proper mechanic would never pass this bike.
I've seen ally frames crack at track days after dents like this. Steel can be bent back and fixed.