I wouldn't think a flanging tool would be needed, but I could be wrong. This looks like a lower door seam repair. You grind away the seam this side while leaving the tailgate 'skin' alone. With the doors I've dealt with, the skin was rusty on the inside not needing replaced, but the seam as well as some adjacent metal needs replaced. You maintain the edge as best you can, leaving the skin alone as much as possible and cut back this side of the seam. I would bet once he hits it with a grinder, its not as bad as it looks.