I know multiple people with clearances who have used marijuana and significantly harder drugs as well and were honest about this on their clearance paperwork. I obviously don't know the specifics of your friend's case but it is not true in general that intelligence agencies (White House may be different or maybe was in the past) must deny you for past marijuana use, particularly if you admit to it early on.

Edit: marijuana use is now common enough that investigators will actually grill you and accuse you of lying if you say you've never used marijuana.