Your history suggests osteoarthritis.
Try:
1. Regular knee quadriceps exercises ( static and knee bending).
2. Ice pack application will help
3.Medications like Cartigen DN and Enzomac forte will help.
Next Steps
Intrarticular injections help but in early stages -1 and 2.
They do not work in later stages.
Health Tips
Gap in knee is normal. More is good.
In osteoarthritis, the knee gap decreases which causes bones to rub against eachother and chase pain and inflammation.
The sure shot cure for knee osteoarthritis (advanced stages is Knee replacement.)
Share your mother's weight bearing x-rays..to detect the stage of her osteoarthritis.