I would like to know the exact reason for zina being forbidden. Also how can i advice someone who has so much desire for zina ?
Zina is forbidden because Allah has explicitly forbidden it in the Qur'an:
ولا تقربوا الزنا إنه كان فاحشة وساء سبيلا
And do not approach zina (unlawful sexual intercourse). Indeed, it is ever an immorality and is evil as a way.
If Allah's supreme authority is not good enough for your friend, trying to find weaker arguments that might convince them is probably just a waste of time.