What would happen? Depends. Are there TWO witnesses? A Confession? If not, zip.
I have seen many PIMO JWs "get away" with almost anything for years by doing the old "deny and counter allege" routine. If only ONE person reports something, the "sinner" denies everything, states someone misunderstood, made a mistake, or is slandering, and demands to confront their accuser.
The long-term effect is earnest honest people who confess, get disfellowshipped, and devious liars rise to the top.