The RMx units use the Corsair "Type4" cables. They're compatible with each other. https://www.corsair.com/ww/en/psu-cable-compatibility
The only issue is the RM850x comes with extra cables over the RM750x. Will vary slightly depending if we're talking about the RMx or RMx (2018), but you might be missing some additional EPS, PCIe, and SATA cables that you would normally get with the RM850x.
Your friend should return the unit to where they bought it from though if it was incomplete. They got ripped off and lied to. Get a refund and buy from somewhere else.