Tile Over Drywall ???

I am doing a remodel and the drywall guy left the shower stud bays uncovered so the tile guy put up paper and cement and is ready to tile. In the kitchen and over the bathroom vanities, he drywalled it. Is it ok to have the tile guy either put up lath and cement or wonder board on top of the drywall as a backing for the tile. He says he doesn't want to rip it out if he doesn't have to. Some guy in a store recommended a sealer ... Red Guard and said if you put that on the drywall (green board) it helps the cement or thinset to bond. The tile guy says that is very expensive and not necessary. He also says that good caulking between the vanities and wall is good for water protection. He says that bitchitaneing that joint is also not necessary. I am afraid that water that seeps through a failed grout line can make its way to the drywall behind and cause problems. Any help or advice would be appreciated.
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I don't like to put tile on dry wall, or green board, in a kitchen or bathroom situation if there is even a remote chance of moisture (or high humidity) getting to it.
Durock over existing drywall will work and would be my preference if you can tolerate the extra thickness. I know of no way to truly seal gypsum board.
Even in areas where no water is anticipated I like to cut the drywall back about 1/2 to 5/8 inch above any counter or other horizontal surface and let the tile extend down to the counter with no drywall behind it for that distance. That way if any water gets under the tile (and it will...sometime) there is no gypsum board to get wet and swell.
Bithuthene is an asphalt derived mastic that is used to apply a water proofing membrane, usually on roofs or sometimes over concrete exteriors. I'd not use it in a kitchen or bath.
RB
Michael Roback wrote:

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