What pressure rating should a replacement safety valve on a compressor have?
Your friend is right. If the operating and safety pressures were the same (nominally, given tolerances), you'd likely have nuisance trips of the safety valve. 

Enough safety factor is built into the tank that it's not a concern to be 10 psi higher with the safety valve. In fact, the tank is probably the same one used on higher-duty models, with the pressure being limited by the compressor motor's abilities.