Made those changes – no change. The php error is specifically on line 75 where the function is called by the hook – still thinks there should be 1 not 3. I tried:

public function cmdu_read_only_message($can)
//public function cmdu_read_only_message($can, $cap, $id)

{
print_r($can);
die();

And the result was 1

So I’ve tested this out and not quite what I wanted. If I attempt to view the first (parent) message, I get an error saying I do not have permission to view the message – what I wanted was to be able to view the message but not reply (read-only). Also, it has no affect on the children of the first message/post i.e. if I click on a reply message, I can view and reply again.

I can see what the issue is, should be if ( (‘send_reply’ === $cap) and not if ( (‘send_reply’ !== $cap)

I have also used wp_get_post_parent_id to get parent ID’s of reply messages to check those as well