Air Leakage

+1

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.

+1
... 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.

Yes, they are Okna 400. The installer was here today, and I have a much better understanding of what is happening now. On some of the windows, the interior stop needed to be removed in order to place some shims because there wasn't a good seal. While it was open, he put some additional insulation in there, replaced the stop and then re-caulked along the seam where the air was coming through. With the other windows, I'll just be putting some additional caulk along the interior stop, which seems reasonable.

I'm glad he removed the stops and insulated them better. Even when the stops are off you should not feel any cold air coming in between the window frame and your existing jamb.