I am in the midst of fixing it my way, but there is probably a better
way. Walk-in shower has tile all around, and the outside of the shower,
i.e., the rest of the bathroom, has drywall. The outside of the shower
where the drywall stops and edge of bullnose tile meet has a corner bead
rusted long ago at the base on one side. I patched once and it rusted
through, so I cut off the bottom 1" and painted what I could reach with
rustoleum primer. I have it open for now, waiting for primer to set
completely. Then, I thought I would caulk a little piece of plastic
over the rest, let it set wall, and pack joint compound in to cover it.
I believe all the moisture came from outside wall, as tile seems in
good shape. The corner bead and the tile are ever so slightly out of
plumb, and I had to cover the bead and waterproof with primer/paint
where joint compound cannot cover.

I painted the condo above us, about 5 years ago, and it had the same
problem to a more severe degree - tile at bottom of opening was falling
off, and the water probably damaged our ceiling. I let the ceiling go
for a long time, thinking I could convince hubby to replace the drywall.
Peeled paint, rusty nailheads. Repaired, primed, painted, hoping
repairs upstairs have since stopped all leaks ) Upstairs had
glass/alum. door, as ours once had, and theirs had never been caulked )