Recently, I have replied in another question (Can a Hindu be married to two women simultaneously, with both agreeing to this?) that a Hindu man cannot have two wives at the same time. This reply applies to men from other communities also, except where polygamy is allowed, such as in Muslim religion.
Now, as per your question, your friend is already married. Therefore, during the validity of the first marriage, he cannot marry the girl, as such second marriage would be null and void. However, if the first marriage has come to an end (such as by way of divorce) then the second marriage would be possible.
To another question (Is a live-in relationship legal in India?), I have replied that two unmarried and adult persons can have live-in relationship and there appears to be no law to prohibit them. However, if one of the persons is already married, then the live-in relationship may have legal complications and is not advisable.
Dr. Ashok Dhamija is a New Delhi based Supreme Court Advocate and author of law books. Read more about him by clicking here. List of his Forum Replies. List of his other articles. List of his Quora Answers. List of his YouTube Videos.