Do not worry.
Try:
1. Knee quadriceps (static and dynamic) exercises.
2. Local ice pack application.
3. Medications like Tab Osiris, Evion will help for knee osteoarthritic pain.
4. Cartilox sachets have sodium hyaluronate, theoretically this can help build knee synovial fluid- (non veg) source.
Next Steps
visit Orthopaedic Surgeon for clinical evaluation and X-Rays.
For your father-in-law share x-rays to discuss ways of treating OA apart from TKR.
Health Tips
Regular knee exercises helps keep knee fit.