Fireplaces, Heating Stoves, Flues and Chimneys - Crumbling Wonderboard under woodstove.

I'm new to any kind of internet forum, so if I make some gauche mistakes, I would appreciate some direction.
Here is my question:
I recently bought an older mobile, and the previous owners had an addition built and installed a wood-burning stove. Since I live in Northern British Columbia, I expect to get a lot of use out of the stove. However, for the installation, the previous owners laid a sheet of Wonderboard directly onto the carpet, and installed the stove on top of it. Apparently, this has passed inspection, and I have seen the paperwork myself. Now for my problem: The Wonderboard is badly crumbling. Everytime I step on it, it crumbles even more. Can I use a cement repair kit to fix this? Thanks for your help.

With the understanding that you need to do this installation over and correctly, you can cut another piece of wonder board to fit around the stove and marry the two with thinset mortar.

Thanks for your help, Snoonyb. That is a practical and doable fix. But won't the new Wonderboard end up crumbling also? The present sheet has areas about 50 to 75% gone from the surface down. Can the thinset mortar be used to fill in the gaps, and won't the bottom board continue to crumble away underneath?

Absolutely, thats why I said, with the understanding that you will have to do this over correctly.