Andrew Young, 40, died after being struck in the unprovoked attack by Lewis Gill, 20, in Bournemouth, Dorset, on November 6 last year.
A chilling video of the assault was released by police.
Andrew's mother spoke of her outrage after Gill was given a four-and-a-half-year jail sentence for the punch that killed her son.
“I saw the CCTV footage in court and you can see that Andrew didn’t cause Lewis Gill any harm," said Pamela Young, 71. "The sentence is an absolute joke."
More top news
Analysis from the Home Office suggests as many as 13,000 people may be in slavery in the UK - much higher than previously thought.
A headteacher has reportedly warned "snowplough" parents are so over-protective that their children end up unable to deal with failure.
The largest horse chestnut tree in the country declared as the 300-year-old specimen at the estate where PM Benjamin Disraeli once lived.