My question involves police conduct in the State of: Missouri

A friend of mine was driving through a city, when she was pulled over. She was not speeding, had valid tags, and had not performed any sort of moving violation. The cop got out of the car, and asked her to step out of her vehicle. He placed her in handcuffs and patted her down. He then put her in the back of his vehicle. She continued to ask him why she had been pulled over and he never responded to her. He asked where her d.l and reg. were and she told him her d.l. was in her purse, in the car, and her registration was in the glovebox. He said he was going to search her car and asked her if there was anything illegal in it. She said no, and again asked why she was pulled over. He searched her purse and car, ran her id, let her go, and told her "If i ever see you in ***** again I will arresst you. He would not give her a reason for pulling her over, searching her car, or placing her in handcuffs. Does she have a complaint against this officer? Or are we just naive to what police officers are now allowed to do...for no stated reason?