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Before a Captain's Mast, you have to understand that that is not a court. It is a single commander that makes the determination of whether or not to punishment him under that non-judicial punishment form. A discharge is NOT actually available in that forum. That would have to come later through a separate process, called an administrative discharge process, so the Captain's Mast is just the first part of the process.
He certainly can make a statement that he did not do any drugs and his drug offenses occurred before he joined, the service. However, while that may help with the punishment issue before Mast, it will be admitting to the Article 80 violation because he will be admitting that he submitted false paperwork to join.
I think his best option really is to maintain silence rather than giving additional evidence against himself.
For him to get an Other than Honorable discharge, they'd have to put him through a board hearing and that would be more difficult for them to accomplish than the Captain's Mast, because it is more like a trial.
He could waive the board to get the General discharge, but his chances of getting an RE 2 are almost nil. If you join the service by giving a false statement to get in, they won't let you come back after you're separated.