NOt receiving Experience letter and relieving letter

Hello sir, One of my friend working as contract employee in L&T technologies is having her engagement/ marriage this month. According to offer letter given to her during joining it was mentioned clearly in that "your work period will be 'this date' to 'this date'". Her contact is over now and she didn't received any extension letter. As her marriage is there this month and she need to settle in her home state she resigned from their saying her contract period is over and she need to leave due to above reasons. Now they are trying to threaten her that you have to serve for one month notice period or you will get negative feedback in your experience letter. She asked its not possible my marriage is there and as contract is over she don't want to continue and there is no such notice period required. I asked her to have patience and talk but now they are going to down level and threatening her that she will get negative feed back and she will not get another job in any company or they will not give experience and relieving letter. I am seeing this as case of mental harassment. Also there is no support from HR or manager or any higher order authority, they all are same. What can be done in this case? How she can get experience letter without affecting her career.