Remodel bath: sub put 6-mil poly on the exterior wall, greenboard over that in the tub surround, and is about to thinset tile directly to the greenboard.

Yes, stop.

Code & moisture issues:

• 6-mil poly on exterior wall is a vapor retarder, not a drainage plane. In a bathroom, you need a proper weather-resistive barrier (WRB) like housewrap or building paper, not poly.
• Greenboard alone is not a code-compliant backer for tile in wet areas. Per IRC P2705.2 and TCNA RH115, tile in tub/shower areas requires a water-resistant tile backer board (e.g., cement board, fiber-cement board, or approved waterproofing membrane).
• Thinsetting tile directly to greenboard will fail. Greenboard is not rated for tile attachment in wet zones.

Correct approach:

1. Remove the poly and greenboard.
2. Install a proper WRB (housewrap or building paper) over the exterior wall sheathing.
3. Use cement board (e.g., Durock) or an approved waterproofing membrane system (e.g., Schluter-DITRA, Wedi) as the tile substrate.
4. Tape and seal all joints per manufacturer specs.
5. Apply thinset and tile per TCNA standards.

Safety note: If this is a structural or exterior wall assembly, consult a pro to ensure proper moisture management and code compliance.

Stop now—redo the wall assembly correctly before tiling.

Stop immediately. This assembly has three code violations and will rot.

1. Wrong Board (IRC R702.4.2 / IBC 2509.2): Greenboard (MR board) is prohibited as a tile substrate in wet areas (tub/shower surrounds). It loses structural integrity when wet. You must use cement board, fiber-cement board, glass mat gypsum board (e.g., DensShield), or coated foam board per ASTM C1178/C1288/C1325.
2. Vapor Barrier Trap (IRC R702.7 / Building Science): 6-mil poly behind the backer board creates a double vapor barrier (poly + tile/grout/thinset). Moisture drives inward (solar vapor drive) or outward (interior humidity) and condenses on the cold poly. It cannot dry. Remove the poly. The WRB belongs outside the sheathing (housewrap/Zip), not inside the stud bay.
3. Missing Waterproofing (IRC R702.4.2 / TCNA B415): Tile and grout are not waterproof. You need a liquid-applied or sheet membrane (ANSI A118.10) over the correct backer board, integrated with the tub flange.

Fix:

1. Demo greenboard & poly.
2. Verify exterior WRB/flashing is intact.
3. Install cement board (screws per mfr, 1/8" gap at tub flange).
4. Tape seams with alkali-resistant mesh + thinset.
5. Roll/brush liquid waterproofing (RedGard, Hydro Ban, etc.) 2 coats over entire surround, lapping onto tub flange.
6. Tile.

If the sub argues "we've always done it this way," they are building to fail. This requires a re-do.