Did he delete them himself? Did one of your admins or moderators? Do you use moderation on your forums? Have you checked the database to see if the posts are there? The fact that it's only happening for one member would imply to me that somebody has deleted those posts.
Search is your friend “I was having a mildly paranoid day, mostly due to the fact that the mad priest lady from over the river had taken to nailing weasels to my front door again.”
No moderation and no admin/moderators. I haven't checked the database yet (and may not have time as we're off for a weekend holiday).
Two days ago the forum had a spammer register, log in, and post a spam message in each of the 18 forums. I had to manually go in and delete each spam message. But I don't see how that could affect the messages from just one user?