I have many tall friends, obese friends and even short and/or skinny friends but no one has ever moaned about the position of armrests and their contribution to discomfort.. All the more so no one has blamed them for contributing to an injury.
You don't need armrests to sit up straight during landings. Simply put feet at on the floor and press your back against the seat back; don't slouch. This alleged injury was not caused by the landing per se; if it was there would be other injuries and damage to the aircraft. The fact there are none means that this injury was probably an exacerbation of a known pre-existing issue and the OP is hoping to garner some money without good reason.
Most people on here have flown hundreds of times and have at one time or another experienced a hard landing. I have experienced two; one so hard the Lufthansa pilot stood at the cockpit door apologising as we walked off the plane. But no one suffered an injury as a result. This claim is without merit.