My neighbor has been hospitalized for 3 months now leaving his cat. Someone feeds and waters it everyday but otherwise it is left alone. Is this cause for concern?

If the cat is properly fed and watered and litter box is cleaned every day I wouldn't worry. If the owner passes then the cat should be rehomed.

If I were you I would tell the person or persons who are feeding him to take him home with them until his owner comes out of the hospital because there could be an emergency, such as a fire, a hurricane, or an earthquake and he will be trapped in the yard without knowing what to do at best. Or at worst, he will be killed instantly if the house's roof caves in because of the earthquake for example. But if the owner dies in the hospital then he should be taken to the shelter or to a friends's or a family member's house immediately.