Fixing shower wall

Assuming your shower it tile check out this site:

https://www.thespruce.com/how-to-fix-water-damaged-shower-walls-4138287

I found a video that might help you with your problem...

If it is rotting through, you'll have to tear out the wall, use cement backer board to make it waterproof, then tile or whatever you are planning on using.

Agree with Sal85902 that the only way to 'fix' this problem is to do it the right way. Anything less than thorough will just be inviting more serious problems.

If the wall was not waterproof, it was not fit for purpose! Since it has degraded, if you rent then call in the Landlord to replace it . If you own the home, then deal with it real soon before it causes wet rot everywhere. Make sure after removing wall (I assume it is a wooden home and not Brick as you may need a builder to help, if it is), before replacing anything that the whole area is dry first!

We had this same problem and we just removed the tile on the rotting wall, tore out the drywall, repaired the wood studs. We used the green board, which is waterproof and replaced the tile. We could not find the exact tile that was on both ends but one a little larger and the same color. We put an accent panel which catches your eye and you never notice the tile on the back is larger than on both sides.