What advice can I give a friend who admits that he purposely fails his promotion?

I recommend one of two things:
1. That he start taking his military career seriously (and putting his best efforts toward getting promoted), or
2. That he update his resume and start looking for a civilian job.

I guess I would have to ask.... Why?

Is it a question of competence (or perceived competence)? Responsibility? Time/commitment?

There are lots of valid reasons to turn down a promotion NOW. There are not a lot of valid reasons to turn down promotions long-term. For a young Soldier planning on an active duty military career, the number dwindles down to 0.

Here's the thing. He isn't fooling anyone. Either he is screwing up so badly and so consistently that those 1SGs now believe he is totally incompetent, or he is screwing up in such a manner that the 1SGs think he is competent but a jackass. Either way, he is demonstrating to those 1SGs that not only is he not ready to lead Soldiers, but he also is not ready for ANY responsibility - to include the responsibility of schools, TDY, moving out of the barracks, or even staying in the Army.

I have met a lot of senior NCOs who were not particularly bright. But very few who couldn't see through BS. And NEVER a group of them, like you see on a promotion board. And every single promotion board I sat on, we DEFINITELY discussed the candidates before and after their appearance.

Your friend is setting himself up for failure not just now, but for months to come. If not years.

I've seen a SM be SPC for a decade - just had no desire to be an NCO or lead. It's his career. At some point RCP will get him and he won't have to worry about the board any more. If he does wish to continue to servie then he will need to show some interest in passing the board and moving up.