Help with tub backboard for tiling

This weekend a friend of the family installed a new tub for us and put new backboard around it. Unfortunately, he didn't do the greatest job with it and now I need to know how to fix it!
The backboard sticks out from the wall 1/4" at the top (which it always did) and continues down straight until about a foot from the top of the tub. At which point it rapidly tapers down to nothing!
We need a relatively flat surface for tileing, and also to make it look finished. My question...how do I add an extra 1/4" thickness to the area? It has to be able to hold up ceramic tiles
Any help???

You can put some thinset mortar on it up to 1/4" thick, let it harden, then put on the tile. Regular thinset shouldn't be greater than 1/4" in a single layer (although you can layer it if you let it set inbetween). The tub should have a tile flange on it, if it doesn't , you may need to do some other things to make sure you don't have a moisture problem. The backerboard (this is cement backer board he put up?) should not come all the way to the top of the tub - there should be a small gap.

I suggest you ask your questions over on www.johnbridge.com you'll get more info on tiling there. Some of the same people frequent this place, but that is specifically on tiling.

Thanks a million! I've gone over and joined that board and gotten some excellent advice I really appreciate the help