Paul Ely, 38, a officer at HMP Garth prison, Ulnes Walton, a few miles from Leyland and Chorley, had volunteered to escort the girlfriend of defendant Lee Campbell across the city centre late one evening last July.
Campbell, however, got the wrong impression and punched Mr Ely who fell to the ground and banged his head on the pavement near to the Anne Summer's shop in Fishergate, Preston.
Mr Ely, who lives with his wife Pauline in Freckleton, near Blackpool, was left in a coma and it seemed unlikely that he would survive.
Doctors operated to relieve bleeding to the brain and a fracture to the skull was repaired.
He still needs constant care and is only allowed to go home at weekends.
Campbell, 22, a former amateur boxer of Fenton Road, Fulwood, in Preston, pleaded guilty to a charge of causing grevious bodily harm and was sentenced to 18 months in prison.
Detective Inspector, Dave Mckenna, said: "These are the catastrophic consequences of alcohol fuelled violence.
"It was one punch which resulted in severe brain damage and has left a man in need of constant care maybe for the rest of his life.
"I would like to warn others that they will receive a custodial sentence and be held responsible for their actions while under the influence of alcohol."