Water infiltration is one of the most serious challenges in building design, for obvious reasons. Simply put, water must be stopped at the exterior before it has a chance to enter the building.
Your intuition is correct: Expanding foam is not a water barrier, and really is not even a draft stop. It is appropriate only for filling cavities in exterior walls and framing to improve thermal performance. Spraying foam along the inside will not correct the problem because it will still allow water to get into the wall. It could actually worsen the situation by trapping moisture in the cavity.
Now let's get to the real issue. If your contractor is advising you to use expanding foam to address a moisture leak, it may be time to look for another contractor, or at least demand that the problem be properly addressed.
- A primary concern is that mold or rot problems may be present in the wall already, or may develop in the future.
- The only proper solution at this point is to remove the windows, allow the wall to fully dry, replace any wood or drywall that might have water damage, and then reinstall the windows using proper flashing materials and installation techniques.
Materials that qualify as true water barriers are commonly referred to as flashing. These are typically sheet metal (galvanized or copper) and membrane products like those by Grace Construction Products. If moisture is getting behind the flanges, it is likely that the tops of your windows are not properly flashed. Sheet metal flashing must extend up (behind the siding) and over the top of the window frame or casing.
The Grace Construction website has drawings of proper flashing assembly for specific wall construction types; it would probably be a good idea to print out the details and review them with your contractor.