false accusation

My friend order a pizza and paid for it. About an half hour later the pizza delivery guy came back to his house and accused him of shorting him $20.00 My friend paid with a twenty dollar bill and received $3.00 in change prior to him returning. My friend told the pizza delivery guy he didn't owe him anything and if had a problem to call the police. The delivery guy called the police and said my friend pulled a gun on him and robbed him. My friend does not own a gun, his son has a b.b. gun that was laying on the table when the pizza delivery guy came back. My friend never demanded any money or pointed a gun at him, although, he was arrested for robbery. My friend who was arrested had his son there who witnessed everything. What can be done?

Eventually the facts will come out. And we would assume the charges will be dropped. But get counsel involved to make sure the police investigate properly.