Like CanGroIt, I too think he'll be okay. During the 25 years I spent in the Air Force we always used a urine test aimed at consumption during the past 3-5 days, so unless DoD have changed the test since I retired, your fiance should be fine.

I smoked occasionally in the years before I joined, but once I raised my right hand, I never touched illicit drugs for all those years. That, IMHO, is what a person owes the system when they join up. If things change in future, and I hope they do, then that's a different thing, but in the meantime, be squared away. Best of luck to you and yours.