This "bar confession" seems really significant for two reasons.
One, it's not the type of thing you'd just casually mention to a normal mate. For him to divulge something like that and show the other video of the wlderly woman, you'd have to assume he knew this "friend" would be open to it. Makes me wonder is this another criminal who is providing the information as a bargaining tool to get himself out of trouble.
The second thing is what we haven't been told about the meeting. I'm sure the conversation wasn't confined to "I know what happened to her" and "have a look at this other rape video". What else was said? You'd assume the first thing the friend would ask is "what happened then?".