His sentencing order known in Georgia as "Final Disposition", will set out the terms of probation. They are specific and numerous. These conditions often include a provision for "random drug screens". I would obtain a copy which should be available through the clerk of the court where he was sentenced. If he fails... he could be arrested, held WITHOUT BAIL and face a hearing where his probated sentence could be revoked in part or entirely! He shold be seeking the advice of a very experienced criminal defense lawyer. look for one in the Atlanta area on this site with "superb" rating immediately. Do not wait until he is arrested. The attorney may be able to give advice that will assist in getting the meeting delayed...wink,wink!
The answer to your question is - pretty likely. If your friend knows he will likely test positive, he needs to talk with an attorney before the meeting. Please let me know if I can be of further help.
M. Jason Rhoades