police is not filing a fir

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 ?

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.