Air Carrier Captain reported encountering mountain wave event resulted in passenger injury;

Narrative

No forecast turbulence on any available tool. At cruise at FL390 when I noticed the airspeed starting to decline. Almost immediately upon suspecting mountain wave (with a loss of about 10 kts) we received one very abrupt moderate jolt of turbulence. It was a sharp up and down much like a wake event. The stick shaker initiated for a fraction of a second and the autopilot reverted to control wheel steering roll mode. We were well above amber stall band by roughly 10-15 kts even with the 10 kt loss. The event started and was over in less than 5 seconds. After the event I received a call from the cabin that passenger seat 7D was thrown against the lav door as she was exiting the aft lav during the event. The seat belt sign was not illuminated. The lav door was knocked off its tracks (ELB filed) by the weight of her body falling against it. Passenger made a statement to the Flight Attendant (FA) like 'that's going to hurt tomorrow'; but refused all offered treatment; including ice; medical assistance; EMT upon landing; aspirin; wheelchair; etc. PIREP filed; Dispatch (XXX) notified; post flight notification to Chief Pilot accomplished via voicemail.