Can they refuse his holiday request?

Hi,my friend asked for holiday in april for 3 weeks time. He is forklift driver at night. There is only 2 of them at night all the time-when one going holiday there is no cover,they just doing what they can and day shift pick up rest. His holiday request has been refused due to manager doesnt want to leave one guy at night for......3 weeks time. Plus addaed that some staff on days is also on holiday. Can they refuse it with that reasons? What is day shift holiday to nights when never covered or long holiday????