replacing tile on bathroom wall

In an effort to replace pipe for shower, the tile was demolished, together with the mesh behind it. After replacing the pipe, backerboard was installed and half of the wall was replastered. The job was never finished. I have noticed that the plaster is separating at the deliniation point between undamaged portion and repair attempt, leaving a deep, open crack, with larger areas below the crack widening and appearing to be gouged out. The original job was completed by a person who was a plasterer in S. Fla. in the early 70's, and I think the original plaster must have been somewhat like stucco, the finish is very rough. What measures can be taken to fix this problem so the wall can be replastered and the tile replaced? The longer it stays in its unfinished state the more likelihood there is that moisture will seep into the interior of the unfinished wall and start a mold problem.