Yea, I will ad some kind of waterproofing to the plaster. The wall is dry tho, we haven't used the shower in about 2 years.Then you are going to need to chop out the adhesive, and then leave the wall to dry for as long as absolutely possible.
You may need to do a similar thing on the other side of the wall as well to be sure to get rid of any of the damp problems.
I would still use a water proofing admixture in the new plaster though, just as a safety measure.