My mother has neck stiffness, what should i do for her?

Many : From just sleeping the right way (small flat pillow) & avoid prolonged positioning or an awkward one or overuse of the muscles with some activity to name a few. Treatment can be otc medication, heat applications and stretching while in a hot shower to avoiding activities that may aggravate your symptoms . If a smoker stop & if a couch potato, consider regular exercise. Massage may help as well.

Cervical DDD: Based upon age, you have cervical degenerative disc disease/spondylosis/arthritis. Like wrinkles of skin, arthritis is a certainty if we live long enough. After age 60, almost all people have arthritic changes. Stiff and painful is likely from inflammation from the above. Try some tylenol (acetaminophen) or aleve if safe for you, salonpas hot patches, ice/heat. If not effective, you may benefit from mckenzie pt.