So that is an excellent question! A party wall is very common in multilevel or multi unit housing. You need to first repair the FireWall with the 5/8" FireBoard. You need to tape the seams and ensure there is no voids anywhere so that air can not move from your unit to theirs.
Then you install for example some Concrete board over top.
If you are replacing the fiber glass shower unit it would go in after the firewall is repaired.
Do you have neighbors that live below you? If so you will also have fire blocking issues around the drain.
If you can post a few pictures of the set up there might be another issues that needs looking at.
DO NOT SCREW THIS UP - these codes are designed to save people's lives.
If you can not post pictures send them to me and I will do it for you.