I think you must be mixing up your wife with your heater. Sounds familiar.
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Can't say anything specific, but there is a temp sensor under the dash (behind a tiny grille at my right knee (LHD) - I suppose that would be passenger right knee in a RHD). This sensor has a habit of falling off when any work is done in the area, and end up dangling close to the air outlet. Or that it gets disconnected from the plug and not replaced.
There are also cases of failing mixer valves.
And then it could be low coolant level - or otherwise air in the system.