What Causes Persistent Pain In The Knee And Popping Sound?

Hi my mom's already 56 and she's having this knee problem. Whenever she bends her knees she hears it pop. The popping sound bugs her. Then when she walks she can feel pain on that knee so she would always limp off or walk with a cane... She always complain that her knee feels weak and sometimes she gets out of balance. How can we solve this problem? Is it the cartilage tear?? Is it already severe? And what medicine should she take? Thank you in advance!