1. This is gross unfair trade practice and illegal act.
2. For this your mother can file a case for compensation in local consumer forum .
3. Additionally a case of forgery and criminal impersonation can be filed against the erring staff.
4. if the police does not act then local Magistrate can be approached for this.
5. Before doing all these do lodge a complaint with police and then send a complaint letter to higher officials of the DTDC. It should work to settle the dispute out of court.