It can result in a coma, paralysis, incontinence... any number of unintended consequences. You need to tell someone immediately about his plans. Forget about secrets and trust... this is his life we're talking about here. Such thoughts should never be dismissed. Go tell someone NOW.
It is possible for a failed hanging to result to a coma. This is because this thought had already occupied his mind and the interruption can result to a coma. However, you can prevent this through counseling and clear that thought from the victim.