How To Get Indian Citizenship, Nepali Living In India From 2004

One of my friends shifted to India with his family in 2004 (at that time he was 18+). After 5 years(at the time of the election), he and his family got an Indian Voter ID cards with the help of Pradhan (Who approved that his family is the residence of his village). In 2013, his family moved back to Nepal but my friend stayed here in India and completed his 12th, and bachelor from Indian college. He has PAN Card (49A) and Aadhar Card (Used voter ID card to get Aadhar card), a bank account created with Aadhar card and an employer id card. He is working in a private company and he never used Nepali documents (except 10th Certificate for DOB). Now he wants to bring his family to India. What is the legal way to get Indian Citizenship? Should he and his family stay here for 12 years to get Indian citizenship or continue with Voter ID card and create other documents? He and his family are very good in nature and they never involved in any criminal cases.