They can both be correct, depending on what you mean. "Revised" says that the accounting director made a change. The reader does not know whether the director changed the report a month ago, or 10 minutes before the report was sent. "Had just revised" implies one of two things. With a full sentence, "had just revised" means that the change was made right before it was sent. You could also use it if there is something else about to interrupt or change the action in the same sentence. "The director had just revised the report when new sales numbers came in. The director has to revise it again." In your example sentence, you have the first meaning. "Had just revised" tells us that the revision happened right before the report was sent.

Neither of these examples are wrong. "The accounting director revised the budget report before it was sent for approval last Friday." This tells us what the accounting director did to the budget report before it was sent for approval. "The accounting director had just revised the budget report before it was sent for approval last Friday." This tells us not only what the accounting director did to budget report before it was sent for approval, but it also tells us when he carried out this action. In this example it is the timing of the action that is central.