Looking into this issue a bit further, it appears, as Julian says, that the later BSA bikes were fitted with relief valves rated at 70 psi or so. However, my understanding is that the relief valve shouldn't be viewed as an oil pressure regulator as such but is better viewed as a blow off valve to prevent excessively high oil pressures being generated. If this is correct then the normal oil pressure measured is purely dictated by the pressure generated by the oil pump which is a function of rpm, temperature and viscosity of the oil. Around 50 psi should be normal at 3000 rpm.
In your friend's case, it would appear that the relief valve is not doing it's job properly as around 70 psi should be the maximum oil pressure measured. It might be worth taking it apart to make sure it is operating freely.