I think that this is not possible, maybe he deleted some of his posts, but the activity of an account is what determines the rank of the account, so maybe it was a bug, i dont think that there is a way that an account can be downgroaded from any rank, it is impossible i guess.
I tried it when i got promoted to Full member, i had 120 posts, and i deleted one, and nothing happened to my account (this happened a few months ago)
No, that's not the way, that activity works. When you have deleted one of your 120 posts and would have waited for an hour, your rank would have gone down to member. There is a delay in activity counting.
Full members, senior members and members can downgrade, when they loose their activities. So, don't try out to delete all your posts, because you would become a newbie, lol.