The friar's desperate plan is a direct result of Juliet's desperation. The friar does prevent Juliet's immediate suicide and in that way his plan is a success. Of course, we know that his plan will eventually fail because Juliet will kill herself at the end of the play. His plan is intricate and complicated. He wishes to spare Juliet and her family the shame of refusing to marry Paris, but he also wishes to hide her previous marriage. Since the friar did not have permission to marry Romeo and Juliet, he is trying to avoid this truth coming to light. If Juliet will pretend to be dead instead of killing herself, he can write to Romeo and ask him to come retrieve her. Then, the two can slip away with no one the wiser.
There are so many things that could go wrong with this plan. Juliet herself wonders what will happen if the potion does not work at all. Of course, it could also work too well and kill her instead of making her sleep. Also, her parents might have discovered the deception. So many things could go wrong with the friar's plan, but the main problem that occurs is Romeo's failure receive the message that her death is merely a pretense.