Some bodging twat did something similar on my Capri years ago. They obviously had rotten arches so chopped off the outer arches, then chopped off the rusty inner arch where the two spot weld together. Then slapped a new arch over the top, lap joined it on with spot welds to hold it on, filled the join and most of the rear quarter panel and painted it.
They didn’t even attempt to replace the inner arch they cut out though so the two were left unattached! Eventually damp and road crap worked it’s way into the gap and then the badly welded seam and it rotted again, bubbling and pushing all the filler out from the inside.
It’s not that much more hard work to do it properly!
I needed whole new rear quarters to restore it this time. They’re huge and cost over a grand each!