Updating posts is not enough. What about threads? PM's? etc..
The only secure way of doing this would really be to restore the database.
Yes, that would update only posts as I stated above. But that is the fasted way in my opinon. Because if he would restore a previous backup, then he would loose all the other posts, topics, new members etc that were accuqired after that backup was made. At least that is how I see it. The OP can do as he sees fit