1. It was not a good step to hand over the registration cost to the builder before the lifts are installed and before the possession is handed over to your father.
2. At this stage I shall advice your father to first take over possession of the flat from the builder against a possession letter to be issued by him.
3. Once possession letter is availed and possession of the property is taken by your father, he should file a complaint case against the builder alleging deficiency in service and unfair business practice claiming installation of lifts with in another 15 days and registration after 10 days thereof failing with penalty of Rs.1K per day, damage and cost. It should also be prayed that the builder should if the builder does not pay the penalty till next 6 months, his Bank account and/or house property shall have to be seized for the purpose of selling to recover his dues on you.
4. If there are multiple people facing the same problem with your father then all of them can jointly file the case before the Consumer Forum which is expected to be decided with in 1 to 1 & 1/2 years.