BRNav
The requirement for BRNav is given in EASA AMC20-4. It requires that the aircraft is equipped with a Nav system that can navigate to an accuracy of +/- 5NM cross-track error (RNP-5) and allows the pilot to select a direct-to waypoint which can come from a suitable current database, or can be manually entered (as in the case of a VOR/DME based RNav). Where the system is based solely on GPS data, there is a minimum spec for the GPS of TSO C129a, and additional inputs such as altitude aiding is required.
The system is mandatory for all IFR flight above FL95 in Europe, so your CJ crew shouldn't legally be able to operate without their FMS (as it's their only BRNav source). Some CJ's have a stand-alone GPS in addition to the FMS (possibly a KLN90B or GPS400). However, the requirement in the AMC requires that cross-track error must be presented to the pilot on a standard deviation indicator within the pilot's normal scan, and if two pilot's are required, both must have this display, so a stand-alone back-up GPS cannot be used to meet the BRNav compliance.
The requirements for PRNav are laid down in JAA TGL-10 and require RNP-1, which is more stringent, and have lots of additional certification hurdles before EASA will approve the required AFM supplement.
Hope that helps.