Walking is a great exercise. Just because you walk slower doesn't mean you're not reaping the benefits. It may just take longer to reach the same benefit, but the active person is always better off in many types of disease situations compared with the inactive person. My mom is 62, has smoked for over 40 years, eats all the wrong foods, never exercises and has a strong family history of heart disease. You can remind her that heart disease is the leading cause of death for women in the US and that in addition to causing death it is one of the major causes of disability.
People often forget that it is disabling and can result in loss of their independence.
Smoking is also associated with increased risk of other diseases, particularly lung disease and cancer. You can tell your mom that you want her to live a full and healthy life. The best weight-loss programs work slowly, over time, and often result in a combination of diet and increase in exercise. Rapid weight loss is often just loss of water weight and can have associated medical problems. People who lose weight rapidly typically regain most of their weight.