for 6 months RP as well as Passport,while on return journey at check in counters they will ckeck RP validity,if any passenger is denied entry on arrival the Airline he has flown must deport back to the point of Origin of the passenger at Airline cost,so check in staff make some fuss.

If the RP validity is current with out expiry definitely he can enter and he will pay fine if he dosent renew