Only a criminal lawyer well versed in immigration law or an immigration lawyer can give you a complete answer to your question. In general if he is undocumented , or even if he has some limited right to be here, and he is charged and convicted of sales he is in danger of being subject to removal proceedings. Repost your question in immigration.
The United States Supreme Court says lawyers must be able to correctly advise clients of the immigration consequences of a conviction. Attorneys who don't understand immigration law are not qualified to represent non-citizens.
Any sales-related offense is considered an aggravated felony, which its the worst type of conviction for a non-citizen. Not only would it get him deported, he would be permanently excluded from the USA. Even attempting to come into the country after an aggravated felony conviction is a new felony that can result in a federal prison sentence.
Even a simple (meaning not sales-related) possession conviction would make him deportable, and he could be denied reentry to the country. In fact, if there it evidence that he uses illegal drugs, he could be deported and excluded, even without a conviction.
As Mr. Kaman says, he needs a criminal defense attorney who understands immigration issues. If he can't afford to hire his own attorney, the judge will appoint a public defender. Fortunately, most public defenders have training in immigration law, but not all private attorneys invest the time and effort to learn it properly.