go to workshops,
record themselves,then seek advice from people they respect and ask for suggestions.
learn to play amusical instrument,here is an example,I have ahalf brother,who was a terrible singer[thin john t], WaddonPete might remember him ,he was a room clearer,he couldnt stay in the same key for one song,however he took violin lessons for some years,the result was an improvement in his singing,he learned to hear intervals better,he managed to learn where to pitch songs,and learned to sing in tune,although not a good singer he became passable,providing he chose the right material.
Singers can learn to train their voices,by using an instrument singing scales with an instrument etc,and then doing breathing exercises,from the diaphragm,to improve their breath control,and then listen to themselves.