I don't think this really is a case of a question where anyone actually needs professional help. There's a few things getting tangled here:
First, the question does not indicate at all that the person OP is dealing with is in desperate need of professional help. We can never be sure in this case, but a change in attitude can have many more benign causes than mental illness. So, I feel safe to assume this isn't one of those cases where we need to suspect mental illness.
Secondly, the question is asking about how to address the breakdown in communication. Which is a good question: it focuses on OP, and what they can do. The question is perfectly answerable without considering a mental illness.
Now, we get to the tricky point: Someone wrote an answer saying the change in behaviour might be caused by mental illness. Note that the writer of that answer is also just self-diagnosing the person they were interacting with at the time: They say they're sure the person was hallucinating and mentally ill, but they don't mention that the person themselves actually admitted being diagnosed as such. This does not mean the neighbour from the question is mentally ill too.
The answer talks about what this, allegedly mentally ill, neighbour did. But what it forgets to mention is how mental illness can influence reaching out to someone, how it invalidates the suggestions done in all other answers, and what OP should do instead. It's basically not an answer.
We've had discussions on questions that do involve people with diagnosed mental health problems before, but this is basically not one of those questions. To see examples of some problematic questions that deal with professional help, but not specifically for OPs: