Thanks, Denver. I, too, would be interested to hear what Jobst has to say about this.
I concur with the others that this is NOT a rim sidewall erosion issue.
This story reminds me of one of our early 1970s Bikecology customers. This teenager, who weighed about 250lbs / 115kg, promptly pretzled the OEM Rigida steel rim on his new Peugeot UO-8. To prevent recurrence, I rebuilt the rear wheel with 14 ga. spokes and a steel Schwinn rim.