When the walls were tiled, how were they prepared? Best practices call for cbu (cementeous backer unit - i.e., cement board) on the walls with either a surface waterproofing or a vapor barrier behind it, lapped over the tiling flange of the tub. Tile and grout are NOT waterproof, and moisture, especially in long, hot showers will penetrate the walls and condense in the cooler confines. This can lead to exactly the symptoms you describe.
Also, is there an exhaust fan, and if there is, is it used? To rid the room of excess moisture, it could take the fan a 1/2-hour after the shower is over or maybe even more, if it was a very long and hot one.
One way to help is obviously, the use of the fan, but also to require they wipe the walls down in the tub/shower area after finishing.
If there was no vapor barrier behind the cbu, or if drywall or greenboard was used on the walls of the tub surround, you are on borrowed time.