Well looks like you are in big dillemma, "Nakhaun bhane din bhariko sikar Khaun bhane kancho bau ko anuhaar." USA laws says that if you  know the crime and you do not inform it, you could be charged with assisting the criminal. So I dont know how much he is selling the dope, meaning if it is some then you are not much in trouble, but if he is moving kilos by day, you better do something about this my friend, and soon I would say. If you really want to help this friend of yours, then you should tell him not to do this, and if he does not listen, may be in a liittle stern way, and tell him you are jeopardizing yourself in this transaction. If he still does not listen, report this to your supervisor. Atleast if he gets caught you have yourself a defense for it. Because some day he might leave a nickel bag at your work place and the cops might get this, and at that point it will be very hard to prove that it is not yours or you are not together in this with him. By the way, is that freind of yours Nepali? just curious. My suggestion, "Do the right thing."