Grout only fills the gaps, as does the silicone. They're not designed to fix anything to anything.
Looks like he may possibly have used a cement based adhesive, judging by how grainy it is and how cleanly it has come away from the plaster. But this also implies that a primer wasn't used. Cement adhesives are not directly compatible with plaster and require an SBR or acrylic primer to adhere to the surface. If it was a ready mixed adhesive, it might stick to plaster, but being applied that thick and being used to fix porcelain, will never dry correctly and could still easily peel cleanly from the plaster.
Porcelain tiles and fixing materials weigh probably in the region of 25+kgs per sqm. The weight limit of plaster is only 20 so there is potential that if the adhesive did indeed stick to plaster, the tiles would possibly fall off and take a good chunk of the skim with it.
Tanking. Part of British Standards now that all wet areas should be tanked. Its always been good practice but now it is official. Water sensitive substrates (such as plaster), if they are not tanked and get wet (which they will do even after they are tiled, from condensation or water passing through the grout (which is water resistant, not waterproof)), will turn to mush and become unstable.
Sorry, but it is a complete rip-out and re-done by someone who knows how to tackle a wetroom, but someone who knows how to tile would be a good start.