Once a President-Elect takes the oath of office on the 20th, he’s the President. The only thing that can remove him (or her)is an impeachment preceding the house and a conviction in the Senate, no if, no buts and no ands.
Your friend, if bold enough to not show up could be court-martialed under at least four violations of the UCMJ that come to mind immediately, probably many more - things like disobeying a direct order, missing a movement, unauthorized absence, dereliction of duty, the list goes on and on.
I’m not related to this case. I was just one of the first to comment much along the lines you did.