Remove the metal corner bead and replace it with Trim-Tex vinyl corner bead.
I like vinyl corner bead because it retains it's shape, so if you hit it hard enough to knock it loose, you can always sneak some glue back under it and glue it on again. If the same thing happens to metal corner bead, then it's dented and you have to replace at least the damaged section, and often it's hard to do that without replacing the whole bead.
The way I like installing vinyl corner bead is to cut it into pieces each about 4 feet long. Mix some white wood glue into a little drywall mud to make it stickier and dry harder. Apply that sticky mud to both sides of the corner, and on the inside of the vinyl bead. Now press the bead onto the corner and push it down with a putty knife all along it's length so that the sticky mud comes oozing out the holes in the bead. Now, do the second 4 foot piece. Collect the excess sticky mud, and wipe the corner bead down with a damp sponge to clean it or any residual sticky mud. Now use some painter's masking tape to hole each piece of bead in place, and let it dry overnight like that.
If you have a lot of sticky mud left, then apply fewer pieces of painter's masking tape, and use that for the first coat of mud on each side of the corner.
I have 66 windows in my building and I'ver replaced the returns on almost all of them, and I wouldn't use anything BUT vinyl corner bead anymore for any exterior drywall corner.
Trim-Tex - The premier manufacturer of vinyl drywall beads
PS:
If you're going to be tearing down the drywall that had Z-brick on it, only to put up ceramic tiles on the new drywall, then you might want to save a step by repairing the mucked up drywall. Just tear all the old Z-Brick off so that you're down to brown paper on the drywall. Now, apply self adhesive strips of fiberglass mesh drywall tape vertically and horizontally, painting over that fiberglass mesh with diluted white wood glue between each coat of fiberglass mesh. As the diluted white wood glue dries, it will bond the fiberglass mesh to the brown paper of the drywall, and in so doing, restore the strength of the missing paper with fiberglass. You can do this trick even if the paper on the gypsum core is completely missing. You can apply fiberglass mesh to the exposed gypsum core, paint over the mesh with diluted white wood glue, and as the glue dries, it will bond the fiberglass mesh to the gypsum core. Apply the next layer of fiberglass mesh in strips perpendicular to the first layer, and paint again with diluted white wood glue. Then you can skim coat over that with drywall joint compound, and install your ceramic tile over that.
