Distressing though it can be to watch someone refuse to eat, this is a normal development of severe-stage dementia. Losing interest in food is often the beginning of the body shutting down.
Force-feeding is never a good idea; your loved one may choke or inhale food into the lungs. Some better options:
Once someone ceases to accept foods, you can follow his or her lead or preferences stated in an advance directive, if it exists.
You can choose to discuss with the doctor the pros and cons of prolonging life through artificial nutrition and hydration.
To keep the person comfortable, offer liquids to drink or swab the mouth with a moistened sponge. Some people enjoy the sweet taste of a small drop of honey or strong, thick juice (like pineapple) on the tongue.