The 1/4" cement board is typically used for horizontal surfaces (couters, etc), and it is fully adhered to a stiff substrate.
For tiled walls in a bath, use the half-inch cement board screwed to the studs, with a tar paper barrier stapled to the studs behind it.
What was the source of the water? Was it a damaged tile (hopefully) that compromised the wall? Or did it come from elsewhere? Hunt down the source of the water and kill it.
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