It is not permissible for a Muslim to sell wine (or any other intoxicating substance) to non-Muslims, because the Prophet (peace be upon him) said:
"Verily when Allaah forbids something, he forbids its price"
(i.e. forbids earning money from it). The Prophet also cursed ten types of people relating to the issue of wine, among them the one who sells it. In addition, selling it would be assisting and abetting people in sin and transgression, and Allaah the Almighty has said (interpretation of the meaning):
...and do not aid and abet one another in sin and transgression. (al-Maa'ida 5:2)
It is incumbent upon you to advise those who have fallen into this evil among the Muslims, that he should repent to Allah the Most Glorified and desist this practice.