A Pakistani friend who needs to go to India was informed (very late, near visa approval) by an Indian embassy official (in Singapore) that he can only enter* India through one of the four major airports: Delhi, Mumbai, Kolkata and Chennai.
* By "enter" I mean that the immigration officers (perhaps the equivalent of "customs agents" in the US) will allow the person to enter the country through the immigration checkpoint of an airport, or be detained at the checkpoint and barred from entering the country. This is not related to "customs" which deals with importing goods, or to being inside a flight that lands at the airport.
Direct flights to and from a smaller airport were already booked, which are now being changed to two-stop flight plan landing at one of the four ports internationally then transiting to a domestic flight to the original destination.
We searched online for any information about such restriction but could not find any. Is this restriction as per Indian law? Where can a reference to this restriction be found?