What to Say

The reason for my membership of this forum for the last few months has been the death, in September 2018, of my beloved wife.
I have noticed several threads where contributors express criticism of what they think are thoughtless comments made to them. Quite often, I am of the view that such comments are made simply because the people who make them, whilst well intentioned, just don’t quite know what to say.
So, can I ask for advice, or warnings about what I should write in a brief note to a Neighbour who has recently been diagnosed with oesophagal cancer. He is a married man aged about 50, with a teenage daughter and a son aged about ten. He has just commenced chemotherapy. I don’t know the precise details of diagnosis and prognosis.
I just don’t want to put my foot in my mouth.