http://book.goindigo.in/skylights/cg...le=C3&page=FAQ
Code:
2. What is required to enter the airport terminal and check-in at the counter?
Travel within India: To enter the airport terminal and check-in at the IndiGo Counter, each Customer must provide a print-out of their itinerary along with a valid photo identity
Important Information: In addition to valid photo identification customers may be required to present the credit\debit card or a photo copy of card used to pay for their booking, at check-in.
The following are considered to be valid photo identification documents:
Valid Passport
Valid Driving License
Election Photo Identification Card
PAN card issued by the Income Tax department.
Photo identity card issued by the employer, being government and reputed private sector organizations.
Children traveling on IndiGo should carry their school identification cards or any other photo identification proof along with them.
Valid birth certificates of infants need to be produced at the time of check-in for age proof.
Customers who cannot provide sufficient identification will be denied boarding.
For all sectors originating or terminating outside India, except to and from Nepal, the passport will be the only form of identification allowed. All Customers, including children and Infants, must present a valid passport and visa (if applicable) at check-in. It is passengers responsibility to ensure appropriate travel documents are arranged and produced whenever required.
Since GoI doesn't require compulsory ID you can get a away with some form of ID with a picture. Worst case you have to get a stamp from the airline that its OK to travel without ID. I'm pretty sure she has a election ID? Its been 17 years since they started to issue those...