Good point Mikey. Here is my original comment to her. By the way she's not paying so if she f's her tile up then oh well.
Try this technique first. Mix water and vinnegar half and half in a spray bottle and spray the wall. Use a scrub pad similar to what you would use to wash your dishes and scrub the tile. Rinse the tile with plain water and then buff dry with a clean white cloth. The vinnegar is acidic and will work if the hazing is not too bad.
If that doesn't work, go to Lowes and buy a product for grout haze removal. Most of these contain muratic acid which is a stronger acid solution and will lightly etch the grout. Mix it following the instructions and scrub the tile. Rinse the tile with water then apply a mixture of ammonia and water to nuetralize the acid in the grout haze removal product. Rinse the tile and buff dry with a white cloth.
Never use vinnegar or any acid cleaner on marble, limestone, or travertine as it will etch or dull the finish.