This really all happened over a simple loose rivnut replacement? I've done those as 15 minute walk-ins before!
There is always a little metal left in the frame after removing a loose boss, but it can usually be shaken out of one end of the tube, spraying crap into the tube to secure it is not a smart move. I think these people should be put on some kind of RBUK blacklist until they up their game a little.
Sadly, just over a spinning riv nut. I guess it was slightly more complicated as it the bolts were stuck in due to the bosses spinning, but that's as complicated as it got.
The bit of metal rattling around sound large and heavy. I reckon they have literally ground the top of the bolt and boss down and punched the remainder of the boss and bolt into the frame, and to disguise this, filled the tube with something to stop it all rattling, but this would seem to have failed.
I'm at a loss as to what can be done to rectify this situation though, other than removing and replacing the down tube, which is not what I really want to have to do, but it that is what's needed, then I guess I'll need to pursue this as an option.