How to fix bathroom tiles falling off
It's a bit difficult to figure out from the photo but it appears that they stuck scraps of tile to the wall with compound and then simply stuck the finish tiles to that with more compound.  NOT A GOOD PLAN!  The finish tiles are poorly supported and also the smooth surface of the scraps doesn't provide good adhesion for the compound.
This is a total tear-out job if ever I've seen one!  I don't see how you're going to salvage any part of this and I'd also be concerned about substandard work on the rest of the job.