My Friend Wants To Marry A Hindu Girl
If the girl is Mulsim but did not declare it, then it is ok. A Muslim is not supposed to marry a Mushrikah. Cultural practices of other cultures are allowed until they entail Shirk which is not allowed, for example, hymns to gods and praying to them during the nikah ceremony are not allowed. They can go with the cultural practices of nikah but not with polytheistic things. For a conscious Muslim, his faith comes first.