Cancellation of approved maternity leave post the leave end date

Below are the details of the issue on a timeline scale: 1. My sister, who is pursuing her PG from Indira Gandhi Institute for Medical Sciences (IGIMS), Patna applied for maternity leave last year September. 2. She was granted a three month maternity leave starting October. She is in possession of the letter which was issued approving her leave. 3. She decided to use the Casual Leave days available to her post that and extended her leave to 5 months. 4. She rejoined in Mar 2017. 5. A few days after she joined, she was informed via a letter that the maternity leave that she was granted was null and void and that her PG term would be extended by 150+ days. 6. When asked for the reason behind cancellation of leave, she was told that it was a mistake and she shouldn't have been granted that in the first place. They are still considering whether to accept the CL days as part of the leave. 7. A point to note here is that she was not informed of the cancellation of the approved leave at any point in the aforementioned leave period. Please let me know what are the options available to her now so that she does not have to extend her PG period by about 5 months.