IF (and that’s a big ‘IF’) the facts are as stated here, it might not support a conviction for desertion. (A)(1) wouldn’t apply if he just left to toke with his buddies and planned to come back. You might be able to fit it under (A)(2) if he was ‘shirking important service’ (guard duty)
” (1) without authority goes or remains absent from his unit, organization, or place of duty with intent to remain away therefrom permanently;
(2) quits his unit, organization, or place of duty with intent to avoid hazardous duty or to shirk important service; or
(3) without being regularly separated from one of the armed forces enlists or accepts an appointment in the same or another on of the armed forces without fully disclosing the fact that he has not been regularly separated, or enters any foreign armed service except when authorized by the United States; is guilty of desertion.”
UCMJ Art 85.