if that small part is poorly designed, i can only imagine what the rest of it looks like...

if you just want a cheap fix (to hold you over until the inevitable shower remodel ) i would just towel up and excess after a shower. if you start cutting out stuff to replace with stone my guess is you are going to uncover a can of worms.

Man, I'd get rid of my dog if he did that to my walls!

But seriously, that looks like an awful lot of water damage for normal use of a 12 year old house. Something else is going on here.. It's got to be leaking during the shower or something.

Looks like it's time to do a total bathroom remodel within a week...

Yep, there's a whole lot of stuff done wrong for that to have occurred within 12.5 years!

Serious answer...you have a bigger issue than a leaky door. You likely have a leak in the pan or another joint between the marble surfaces. The moisture is at the subfloor level and wicking up into the drywall, trim, etc.. you can see the efflorescence on the grout lines where moisture is wicking up through the grout joints.

I'd start with doing a pan test, remove the drain cover and seal the drain with a test ball. Fill the shower pan with water up to just below the first joint between the pan and the next panel or trim piece. Mark the waterline with a piece of tape or pencil and leave it for an hour and see if the water level receeeds.

You can try cleaning out the joints in the cultured marble and recaulking the whole thing. It's a painstaking task, to remove all the nasty old caulk. Moisture will likely seep from the joints once you remove the caulk. The moisture needs to dry out before caulking.

I see a shower remodel in your future. And it's probably necessary in order to remove the wet, rotten material in the walls that you can't see.

That amount of damage indicates you don't have a quick fix. There is just too much going on. You should consider a complete tear out. Get in there and fix the problem behind the walls or in the pan area.

I'm just now looking at the pictures - they don't show up at work but do at home.

To put it in scientific terms, your shower is completely borked due to a lack of or improperly installed waterproofing. There is no quick fix to this other than not using it any more. Might have to rework a bit of framing once it gets demo'd out, might not. It all depends on what you see when that wall is opened up.