When it comes to keeping navigation accuracy RNP-1 is logically more restrictive than RNP-10. The requirement is to be able to meet a Nav. accuracy of 1NM or better for 95% of the flight-time in RNP-1 airspace, whereas in RNP-10 airspace it is 10NM or better. What your friend probably meant by RNP-10 being more restrictive is the equipment required to continue flight in RNP-10 airspace. Since it is mostly in use over oceanic airspace, you need two independent long range navigation systems. On the A320 it would be two FMGCs, two MCDUs and two IRSs at least, without GPS you would have a timelimit of 6.2 hrs since IRS alignment and 5.7 hrs since last FM radio update. Beyond this time your estimated position error is expected to exceed 10NM. If GPS is installed there is no time limit.
In RNP-5 or B-RNAV (european airspace) you can continue flight with one FMGC, one MCDU, one IRS, one VOR/DME or one GPS receiver for FM navigation update. The above systems requirements are from FCOM 2.04.51.