The typical repair for this is to cut a hole in the frame rail, weld in another nut the weld the piece you cut out back in.
A friend had the same problem and he cut the hole to the same size as the triangle shaped hole to access the dogbone bolt.
Then after he re-welded a new nut he just covered the hole by using silicone and the same plastic cover that covers the dogbone access hole. This way if it ever happens again, he can just pry off the plastic cover.
Happiness isn't around the corner...
Happiness IS the corner.