To vindicate himself I would personally let him PM the girls boyfriend with the chat.

But do you really want to get involved? As an impartial bystander you have the probability of a mixture of 2 outcomes:

1. Clear your friend by allowing him copy and paste access to said messages. This ensures your friend isn't called a liar for something he knew was true yet stupid to write. The problem with this is that your description of the situation sounds like you have a tight social group so this may end up breaking up your circle or at least changing the group dynamics dramatically. Also her bf might want to dish out retribution of sorts which probably wont be nice.

2. you keep the messages to yourself. You let your friend take the fall for what you have been doing though he shouldn't have posted it on that chaps wall. Social dynamics will inevitable change because you're sacking off this girl, so she'll go cold, prob with her friends

Your choice, sounds like too much drama for me tbh