Muffler making noise and mechanic didn't fix it?

My brother just bought me an infiniti but he says the muffler needs to be fixed so its been at the mechanics for a few days now. He just picked it up today, but said the muffler is still making noise and he doesn't see a difference. He plans on bringing it back to the mechanic tomorrow and having him fix it again. What if the mechanic refuses? does he have to give the money back or does he need to be sued and is it costly to sue somebody?