Maybe the student isn't giving you accurate information. For instance, in your original post you stated "When they go to the Forums page in their course they can see some posts but other posts have "Subject (hidden)" in the subject field and "Author (hidden)..." but now the student indicates it is a Social forum, not Q&A.
The problem is that the Social forum is only available when the Social format is applied as the course site's format. Ergo, the site is one, big, forum. Period. Below is a screen capture of the types of forums available for sites formatted as Weekly or Topics:
And, as the other folks who replied pointed out, the "hidden" message the student relayed to you is associated with the Q and A forum.
Further, when the student stated to you that they have to "go to the Forums page in their course" - that could only be true if their course site was Weekly or Topics format, because the Social format is only a single forum... there are no other areas to navigate to, and the type of forum cannot be changed (as far as I can tell, when the Social format is applied).
So, these could be two separate issues. Have you checked the permissions to determine if the instructor has tweaked any of them for the forum or forums in question? Whenever I believe I may be getting misinformation from someone (student or faculty member), I often ask them to drop by my office and demonstrate their difficulty. Ninety-nine percent of the time it is user error due.
Also, have you tried logging in as the student to replicate their problem (using the "login as" option)?
Please let us know when you do get to the bottom of this - the solution may prove useful to many of us in the Moodle community.