Passport Related Issue

My friend is facing trouble. He was diagnosed with Mental health illness sometime back for which he is on medication and recovered almost completely. However, his doctor suggested that he will have to continue his medicine for a lifetime so that symptoms do not relapse. He wanted to go on holiday abroad and his office is also keen to send him abroad for meetings. His doctor also gave him permission to do so as he is stable now. But his parents are just against it and they have taken his passport and not giving him back. In fact, they are saying they have given his passport to a lawyer and has filed a legal case against him. If his doctor is saying he is fit to travel, then is it legally right to keep someone's passport. Is there any section under which they can sue him as per our Indian law.