Unfortunately, it's not that uncommon. His frame is probably built by Lippert Components, who make most trailer frames for nearly all the towable RV manufacturers these days. Lippert has had a few issues over the years with the frame breaking around the pin box. There's probably not much that Keystone or Lippert will do on a 10-year old rig, but it might be worth a call to either one. I will get you the customer service numbers tomorrow, if I don't forget.
Otherwise, just plan on having someone drop the bulkhead (plastic/fiberglass covering on the bottom), and weld in some bracing on the pin box. It's not big of a job, other than usually the trailer is not near a welding shop when this needs to be addressed. Sometimes you can find a good, mobile welder in situations like this.
Casey
1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter