Flooring Tile - Backerboard Over Existing Cement Floor?

In the middle of a bathroom remodel where I am doing part and a contractor is doing part.

The onld cement floor is crack and no longer level. The contractor said that the floor feel solid and I should just use some leveler and put Hardibackerboard over it.

Questions:
1) Is he out of his mind?
2) Would doing that cause to much of a elevation in the floor for the toilet to fit back in place?
3) Am I worrying too much?

THis is an example where multiple threads can lead to alot of correct advice being given for a particluar situation, but all is not as it seems. From seeing previous thread, this is a second floor bathroom, therefor you do not have a slab, you have a mud bed. The mud bed since it is cracked should be removed. If it was cracked demoing the tile, and there is no movement on the crack, then a crack isolation membrane can be used after filling in any little divots, etc caused by removing the old tile. If the tile was cracked, and the mud bed underneath was discovered to be drcked under the tile or if there is any verticle movement along the crack whne you walk or jump on it, tear it out, then add plywood to build up the height, which is usually about 1 1/2" or so, and then cement board. You cannot add any cement board over a slab or mud bed. So to answer your questions,
Is he out of his mind?
If he is a relative, then he is just misinformed. Otherwise, yes, far out to left field, going, going, gone!!!
Would doing that cause to much of a elevation in the floor for the toilet to fit back in place?
Since his idea won't work, it's a non issue. You can build up with plywood and cement board once the mud bed is removed to the correct height, and if it ends up higher than the flange, you can have the flange replaced and reset, use flange ectenders, or double wax rings.
Am I worrying too much?
Given the validity of his idea, I'd have to say "NO"