This seems complicated but it’s actually incredibly simple.
In pre-1974 units, the boundary between lot property and common property ran down the middle of the external wall, meaning the windows and balcony doors were lot property. Unless stated otherwise on the plan, the windows are probably the lot owners responsibility.
Tinting them in a way that is not in keeping with the rest of the building is a whole other issue.
However, if the original strata plan says the windows are, as they claim, common property, then you can offer them two options: either they reinstate them to their previous condition at their own expense (under regulations about altering common property without permission) or they accept an exclusive use by-law (drafted at their expense) giving them the responsibility for maintaining and repairing them.
If they are common property, this is an important issue that should not be allowed to slide because if they sell and their failure to take responsibility for the windows isn’t established in EC minutes etc, then the new windows WILL become the responsibility of the owners corp rather than the new owners.