Answer
According to our understanding of the query, your friend married a non-Muslim girl and he did not divorce her. Your query is whether his marriage to her is still valid or does he have to divorce her?
If our understanding is correct, then if your friend married a woman who is not a Christian or Jew, then the marriage was not valid in the first place. There is no need to divorce her. If he married a Christian or Jew, then the marriage is still valid. If he does not want to maintain the Nikah, he should release her.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai