A fixed elbow will eat the force of a strong stroke that would otherwise go into an elbow drop. For the most part, the triceps tendon will get most stressed and could even get damaged...which takes super long to properly heal. This strain on the triceps tendon can cause some inflamation in the area and this is what really causes flare ups in previous injuries in the elbow joint...even if the affected ligaments don't seem to be involved in stroking, or really, decellerating the stroke with a fixed elbow. So ye, believe him lol.
Another factor is that pain and how it is perceived is weird in the body so they may have a strained triceps tendon and attribute that to their old familiar injury. I know I was pretty surprised to find out that plantar fascitis, on the very bottom of the foot, was causing the radiating pain on top of my foot which felt exactly like a lisfranc sprain I had years earlier. Pain perception can be misleading.
Either way, injuries from pool strokes, especially big decellerations eating up a lot of force in a fixed elbow position are def possible. Even Bob has mentioned on these forums that he plays a J stroke with an elbow drop because not dropping hurts. I'm sure that gets more and more true as we get closer to OP's friend's ripe age of 82.