Special Marriage Act

Sir, My son aged 31 is a backpacker and travelled through South East Asia and South and Central America. We are Hindu Brahmans. He wants to marry a girl from Chile (South America). She has stayed in India for 2 moths earlier and for extension of visa went to Shri Lanka and came back to India and then went to Nepal to get her Visa extended. They are travelling in Nepal at present and will come back to India in July 2018 again on a two months Visa. It is not decided yet if she will change her religion. She is catholic. We intend to get them married in August 2018 in a simple religious function.What proofs she will be required to submit from her country? I know about requirement of thirty days stay prior to applying for marriage registration in India and then further 30 days period for confirming no objection. For this she can get 6 months Visa. But she wants my son visit Chile to meet her parents also. So can we carry out religious marriage now and register after they again come back from Chile? What papers she needs from her country?