Missing or decaying teeth make it difficult to eat. Poorly fitting dentures may cause pain so the person avoids eating. Alcohol is an appetite depressant. Lack of enthusiasm for grocery shopping and food preparation Poverty. No money to buy food.
Depression is a major appetite depressant.
So is loneliness and lack of meaningful connections with other people. Some older folks have given up on life and resigned themselves to their approaching deaths. All of these psychological states are treatable. Arrange to have your relative evaluated by a physician. If you suspect depression, include a psychiatrist or other mental health counselor in the process.