Cat is alone all the time

My neighbor's cat gets an hour minimum every day, sometimes a couple of hours of human interaction. My neighbor is never home...and I mean all day and all night, all the time. He comes back during the day just long enough to spend that hour or two, and then leaves again until the next day. There used to be two cats, both of them elderly (16-17 years), but one of them died earlier this year, so the only remaining one is alone all the time.

Is this something that as a concerned neighbor, I should be worried about? I love animals, and I hate to think this one is depressed and lonely.

If you are on good terms with the neighbor, you might strike up a conversation over the fence and express your concern.  The other old cat may well be depressed and probably does sleep a good part of the day, but this would be an ideal situato\ion to see if he would be interested in saving another senior cat to keep this one company.  The seniors usually do get along well.  Alternatively, you might offer to go visit the cat daily if he wouldnt mind since you are home and he is not.  I'm sure the cat mourns for the other cat, but I'm home most of the time and my senior cats sleep in the bedroom most of the time whether I am there or not there.  One goes out for his daily stroll in the fenced yard.  The other doesn't.  They all sleep with me, though, and it's our favorite time

As a neighbour, I don't think it's your place to do or say anything, unless you two are on friendly terms. As for whether the cat is unhappy, I think it really depends on the cat. Some cats do find in solitude and others don't.