How can he move out and not get in trouble at 17?

My friend is 17. His parents told him he can move out and live in a homeless shelter for all they care. so the other day he told them hey im leaving im moving in with so and so and his parents flipped out on him. they refuse to let him leave now even tho they told him he could. things got so bad at his house that his grandparents had to come and get him and he’s staying with them for a few days hopeing that things will cool off at home. he wants to get emmancipated. They people he planned on moving in with had him a job that pays pretty well and they were gonna help him pay for things as he needed them. His parents wont let him get a job because they dont want him to be able to have the money to support himself. how can he get out of there without getting in legal trouble? could he run away and the cops not do anything about it since he’s 17? btw, we’re in the state of Wisconsin. what can he do?? he’s not gonna make there much longer! they wont let him get a car, his liscense, a job, a phone, anything! he’s not allowed to go anywhere. he cant use the house phone. he’s not allowed to hang with friends. his parents are insane! is there anything we could do about mental abuse? please help! :(