police is not filing a fir

good evening, few days ago my friend got struck in a roadrage and when he thought of doing a fir in police station they are not allowing him to do a fir instead they slapped him and told that you should go to another police station in your locality don't come and disturb in nagpada police station again.... I'm not sure who did wrong but.... according to me I don't think police or any inspector can just hit a person or slap him any sections against them or can I file a fir against them this should be not tolerated
please give some feedback,
thank you....

just give a complaint in written form about the incident to the nearest s.p

At least seek information through some other applicant outside Mumbai to such PS  under RTI and get a CCTV footage mentioning date...time...from to...so they can be cautious next time.

Whether your friend has submitted written complaint to PS and higher police authorities ?

You should file complaint in the court against them so that they should be punished and not to do such type of act in future against any one. Feel free to contact at sunayana122@gmail.com for further assistance.

In legal terms, the police is to register the FIR in case of any cognizable offence within their territorial jurisdiction. 

In your case, your friend could send the complaint to the SP or Commissioner of Police. If the concerned SP finds substance in the complaint, he could either investigate the matter himself or direct its sub-ordinates to register the FIR and initiate investigation. 

If all your actions go in vain and the authority illegally refuses to lodge your FIR, your friend could file a complaint before Judical Magistrate/ Metropolitan Magsitrate under sec 156(3) CrPC directing the police to register the FIR.