Back buttering is a trick used for tiles of uneven thickness like ungauged slate or handmade tiles extremely large tiles or for tiles being installed on an uneven surface.
Tiling very uneven walls.
The board will create a perfect backing that will make setting tiles on thinset look like a walk in a park.
There is also a spot near the window trim where the wall appears to be built up too much enough so that the tile could end up proud of the trim which i m pretty sure won t look so hot.
If you put a 48 straight edge up against the most sunken areas there s a gap of almost a 1 2.
Preparing an uneven wall surface plastering.
One very big problem that not many people realise if the wall isn t really flat a brick joint pattern will show off every out of plane point in the wall by virtue of sticking out a half tile from the tiles above and below it.
Being that she s not going to be there very long that might be fine.
If you re tiling over an uneven section of wall the corners of the tiles may stick out slightly from the rest of the tiles.
If you don t think you can trowel a coat of mortar to get it even withing about 1 4 the tiles will further even it out when set i would investigate a possibility of mounting a cement board onto the bricks instead.
The problem is that the wall is uneven.
The mortar is spread.