one of my frns wife is in relation with another guy,?

Question

if he wanna divorce from her wat step he have to take,
their marriage is not registered as per hindu law,
n when they get married girl was under age.
any legal thing he can do pls suggest

Answer 1

i am assuming that both the parties are hindu so they must be goverened by Hindu Marriage Act, 1956.
section 13 of that Act provides few grounds. amongst those grounds ground 1 (i) reads as follows:
1) any marriage solemnized, whether before or after the commencement of this Act, on a petition presented by either the husband or the wife, be dissolved by a decree of divorce on the ground that the other party-
(i) has, after the solemnization of the marriage, had voluntary sexual intercourse with any person other than his or her spouse.

your case seems to be falling under that ground.
even if the girl was underage she had an option under Section 12 of Hindu Marriage Act to seek a decree for annulment for the marriage.

Answer 2

It seems that the marriage was solemenized under the Hindu Marriage Act, 1956 and the parties were Hindus at the time of their marriage. Since the girl was minor at the time of marriage, one can seek anulment of the marriage, Or the boy can seek divorce on the ground of adultery, or the parties can file a joint application for "mutual divorce".

Answer 3

If he is not legally married then there is no grounds for a legal divorce.