The problem is that the wall is uneven.
Tiling very uneven walls.
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.
The board will create a perfect backing that will make setting tiles on thinset look like a walk in a park.
Being that she s not going to be there very long that might be fine.
Preparing an uneven wall surface plastering.
The mortar is spread.
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.
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.
One of the best ways to ensure the surface of a wall is even is to plaster it.
Plastering a wall can be a super straight forward task but if you have no prior plastering experience it may be worthwhile getting a professional in to help achieving a perfectly flat surface for tiling is essential and this isn t always easy on your first attempt.