How to make free calls on BSNL landline from 9 pm to 7 am

BSNL does offer the free calls facility from 9 pm to 7 am to all landline customers. You can avail this option without the need of any separate activation procedure. Your friend may be facing some other issue with it.
I would advice you to opt for detailed billing from the BSNL by submitting the application to that effect. The service is called itemised billing. This will let you get the bill with full details of the call charges billed to you along with the called number, time of the call, units and the charges for the same. In addition, you can also maintain a record of calls made in a separate notebook so that you can compare it with the bill. If there is any discrepancy, you may place a complaint that you have been charged for the caqlls made after 9 pm and before 7 am.
Kindly check with your local exchange whether the offer of free night calls are availabe for the circle. As far as my information goes, the service is available across the entire nation. There are certain pre conditions to the offer. It could be so that calls made to non BSNL numbers is charged, while those within the BSNL nerwork ( both landline and mobile) are free.
Also note that though the actual calls are free, you will be liable to pay the plan charges. This will be in addition to the rental charges.

BSNL's plan for the free calls between the after hours are limited to some states and specific number of cities. So based on those specific cities and the states, other calls will be charged. Do note that monthly basic fee will be charged as it is. And this is going to be in addition to that rental fee.

If your friend has made the call to the city or the state outside the offering, Then that call will be charged based on the outside local or trunk call. If your friend is in village then tandem exchange rules are going to be different for these plans. Most of the tier 1 city plans does not apply to the tandem exchange offers.

I suggest checking out the plan details and based on that you can then see if the calling to specific location would be free during the after hours. The plans for after hours need to be separately activated and you have to make sure that your bill mentions such after hours call details in the bill. If not make sure you clarify with the BSNL office.