Hi Robert,
Afraid I can't help you out directly with replacement adjusters right now. They do appear on eBay... but only occasionally. However, I've frequently found them in the discarded parts bins at old local bike shops. And, they even more often will have the simple one-piece plastic ones from later Peugeot, etc., frames (for real cheap) if you just want to fit a set of those on the bike as a temporary convenience.
PS: ~ I find it hard to imagine how an over-torqued axle (?) or even a loosened adjuster could jam the drive train, unless the adjuster bolt had somehow completely unscrewed and dropped parts into the rear derailleur cage.
Those old Prestige rear derailleurs (if that's what the bike had) did become a bit overly flexy with age. My guess would be that he had just dropped the chain off of the small cog so that it had suddenly wedged between the freewheel and the dropout (or adjuster)... and perhaps that may have yanked the end of the adjuster out of place - giving him the impression that this was somehow the cause.
Anyway, Good Luck! Bob Hanson, Albuquerque, New Mexico
Robert Brooks wrote:
"...Unfortunately, while riding one day, he torqued the rear axle enough that the rear-drop out adjuster broke sending him over the handlebars. I couldn't really get a clear explanation as to what he was actually doing at the time..."
Robert Brooks
Seattle, WA USA