I don't know, to do it properly he'd have to scuff and re clear the entire quarter. This being his first time he might also have to deal with sanding scratches, fisheyes, runs, dirt in the paint, and maybe delamination of the clear later on.
If it were me I'd rather try to save the original paint and just deal with carefully straightening the panel.
There is also a middle ground in which he could just paint up to the body line.
If it were my truck, I'd definently cut the access hole and carefully knock it out.
Alternatively, he might be able to get an inflatable back there through that front body plug and pop it out that way.
I understand that idea but I really think that cutting o hole in that inner panel will cause way more grief than try to match the paint.
He only needs the paint to look good enough up to the body line.
He doesnt need to do the entire quarter.
That paint that is in or near the crease in the metal is going to pop loose anyway.
You can screw up the paint over and over again without even getting near the difficulty of trying to weld that access panel back in place.
Plus. Now he has to buy a welder and a hood and a welding jacket and learn how to weld sheetmetal while laying underneath the truck.
Granted, I am looking at the damage on the worlds oldest living samsung J3 so I cant tell if the quarter is damaged.
To me it just looks like the lower panel below the body line.
If I did decide that I needed access to that area I would cut the spot welds and seperate the inner from the outer way before I thought about cutting a hole in it.
Spot welding sheet metal with a crappy welder is much easier than trying to seam weld sheet metal with a crappy welder.