It seems to me that there may not have been adequate investigation.
Even with a waterproofing additive I would not rely solely on grouting (or regrouting) as a satisfactory preventative measure where there are water penetration issues.
Water penetration of the unit below could be due to:-
1. Leaking pipes (water supply &/or drainage),
2. Failed waterproof membrane (improperly applied or damaged)
Prior to tiling there should be a properly designed & applied code compliant waterproofing system forming a continuous membrane applied to the floor & walls of not only the shower recess (including any hobs, dips or recesses) but also the entire bathroom / en-suite floor. While wall areas (other than the shower 1800mm +) are only required to have waterproofing to a relatively small height adjacent to the floor and other horizontal surfaces, it is generally best to waterproof to a greater height, such as door height, because some occupants can splash water all over the place.
Any pipes or wastes penetrating the membrane must also be properly waterproofed at the junctions where they penetrate the membrane.
While there are some products (eg. a particular paving sealer which I probably shouldn’t name) which can do a reasonable job of waterproofing wet areas when applied as per manufacturers recommendations to the tiled surface this is not the best approach.
I suggest that the problem be properly investigated before acting.