Grout (with the possible exception of epoxy) is never waterproof...sealed or not. Drywall is susceptible to moisture and, the "greenboard" was used in years past on tub and shower walls. It was touted as "water resistant" because of a waxy covering on the face...but was about "two spits" more resistant than standard drywall and is no longer allowed for this purpose.

Regular drywall can be used in this application if it is covered with a Surface Applied Membrane such as Kerdi. I doubt this was the case here. The minimum that should have been done was to staple roofing felt or plastic over the studs to prevent vapor from transmitting to the studs and damaging them. By removing the cover I mentioned above and using a flashlight, you may be able to determine this, and what the backing is. The differences between drywall and cement board are pretty obvious.

Will the job last? Anyone's guess...but it lasted three years so far, so it could last 20 years...or it could fail in six months. What you do have working for you is the tile size. With that size, there is less grout areas for moisture to penetrate. Keep an eye on the caulking at the vertical and horizontal tile junctions and use a pure silicone caulk in those areas. If they were grouted, that's not good. It should be removed and caulked.

Another tip is to towel the tile down after use. Keep it dry and it will make a difference....if you can convince the renter to help you out here.