Missing calcium sand-lime brick - substrate tiles

Hi, in our guest bathroom (new solid construction) the connections for the bidet were set incorrectly (too far apart, for a different model) and have now been "corrected" for the Villeroy & Boch Subway 2.0 Compact. The situation left by the HSK guy is as shown in the picture; he knocked off a lot, leaving little sand-lime brick and a lot of foam. The tiler said he often encounters this kind of situation in old building renovations, applies a good amount of tile adhesive, lets it set, and can then tile on it. We find this questionable if a bidet with its bolts is barely fixed in the sand-lime brick and the tile that the bidet hangs on is glued to a substrate of 4cm tile adhesive and 10cm construction foam. I would like to reduce the construction foam to create a load-bearing substrate for tiling and let the wall "regrow". Spontaneously and without much knowledge of masonry and concrete, I am thinking of "small reinforcing irons," like bolts or threaded rods into the existing sand-lime brick at the top and sides, possibly some kind of formwork (or cutting the existing foam as formwork or foaming some additional "formwork") and then concreting. Is this sensible/feasible? If yes, what kind of concrete/mortar can I use for this, or are there better ways to create a wall again that is not elastic? From what thickness would, for example, a small Wedi board be stable enough?

Uiui at first glance, at least color-wise, it looks like an archaeological find in Egyptian. The bidet will probably be hung on a standing mounting element and thus would have virtually nothing to do with the wall. The tile adhesive bonds with the masonry and is certainly sufficient to hold the tiles. These, in turn, do not have to hold anything. In this respect, I see no problem in following the tiler; your intention therefore seems unnecessary.

No, as already written above - the bidet is to be screwed with 2 bolts into the calcium silicate brick (hollow brick!), which you can see at the top - no mounting element. The heating engineer would do this with injection mortar. It then hangs directly on the wall or on the tile, so the tile must be supported from behind in a stable and rigid manner.

I see a mounting hole in the stone on the right and on the left a foam insulation mess and thus questionably stably embedded a small metal bracket, both below the supply and drain connections. The tile would, in a way, only connect both anchors as a common washer. The weak point, in my opinion, is therefore less the backing of the tile than the foundation of the small bracket, which is not really seriously discernible here. Injecting more material under the tile probably protected the tile against breaking – but how is this measure supposed to stabilize the crucial small bracket?

Hi, in our guest bathroom (new solid construction) the connections for the bidet were installed incorrectly (too far apart, for a different model), and have now been "corrected" for the Villeroy & Boch Subway 2.0 Compact.
The situation left by the HSK guy is as shown in the picture, he chiseled away a lot, leaving little sand-lime brick and a lot of foam.

P.S.: Bidet in the guest bathroom, I'm impressed. However, I would have chosen a different bidet model here rather than undertaking installation-critical remodeling measures.

Maybe I don’t fully understand this, but I consider the project itself to be questionable, since the wall block was already considerably fragile due to the necessary openings for cold/hot and wastewater, even without the additional chipping out. There are very flat mounting elements (about 4cm), which sometimes don’t even rest on the floor but are rather screwed to the wall higher up on rails. I don’t see exactly in the picture where the screws or threaded rods you planned should be inserted into the now broken wall so that they get sufficient hold. Even if you now make something with concrete, this new concrete block won’t connect with the upper wall block or gain sufficient long-term strength downwards, where various pipes run. The toilet next to it will probably have a front wall, so it might be possible to also extend this to the bidet. Maybe you’ll send other pictures. If necessary, a few tiles will just have to be removed again. I also wouldn’t know how a threaded rod or bolt could get any hold in a slapped-on amount of tile adhesive.