Windows on Washington wrote:
Removing the casing is not included during a " basic install" whether interior or exterior...needs to be stated in contract. There are adjusters that can tighten, align the windows. Looks like the 400 series were installed?
These are 400 series windows as evidenced by the lack of the mortised damn leg.
If the customer does not want to remove the interior stop, they can caulk the gaps for proper air seal.
The exterior wall in this particular case is not the envelope layer so spray foaming in there is not going to make any appreciable difference as compared to interior caulking.
... I'd also clarify to the OP that those stops did not need to be removed in the original install in order to insulate properly. It was done from the exterior which is perfectly acceptable, it just was not done thoroughly enough evidently. A concern for the installer as it pertains to this issue whether during the original install, or now during service, is RRP. You can't just start ripping apart 100 yr old painted trim without proper containment. Wow's suggestion above would be an effective and simple fix at this point.