absolute proof isn't the point. Yes, it's possible tom is not potripper. It's *possible* no cheating has occured.
We know it is etremely, extremely likely that cheating did occur. We know it is very likely Tom is a guilty party. We know it is extremely, extremely likely that Sief is: a poker player, has met Tom, and is friends with Tom.
Given this knowledge, it makes it *more* likely that Sief is guilty of his accused cheating, than it was before we had this knowledge.