First let she send her resignation letter to the bank authorities, they will give a reply to accept her resignation or not and also about the bond details, after which you cn plan further legal action on this.
In fact let she join the India posts appointment without relieving letter stating that the bank authorities are not giving the letter for this reason, by enclosing their letter to support her request for joining them.
After that wait for the developments and then plan to initiate appropriate legal action as necessary as per the prevailing circumstances