Builder delaying property registration after taking registration amt.

Hi Sir/Madam: I am putting this question on behalf of one of my relative. He bought an apartment in Ghaziabad back in the year 2009. He was given possession of the home in 2014 or 2015 along with posession letter. Many residents of this apartment society has not registered their flat as they came to know from somewhere that the builder does not have few certificates, completion certificate or occupancy certificate. Builder's representative on the other hand said otherwise and asked my relative to get the property registered and had taken the full amount (registration and stamp duty amount) from him in around mid of 2018, but now keeps on delaying the registration by giving one or other excuses. Recently after repeated reminders, builder clearly says they don't have fund right now and threatening that they are atleast letting the residents stay in their homes without registration as it is illegal. May I please know what my relative can do in this case.