Re: Need help
From your description, the striker plate was cosmetically rusted, but when they tried to replace it its bolts broke and removing them caused the weld nuts to break free and drop down into the rocker (the rest of the vehicle is not badly rusted).
If they cut an access hole in the inner wall of the rocker panel (visible from underneath the vehicle) it should provide them a way to replace the lost weld nuts (and get the broken off ones out so they don't rattle), and after they weld the hole closed should not be visible from outside. A good body man should be able to do that in a couple of hours, max.
Just be sure they coat the result (on the inside) well with corrosion protection, via some access hole they could drill and cover with a rubber plug, to make sure the weld doesn't rust later (weld tends to rust faster than regular metal).