Great question! I have mine set to automatic. When I first bought my Air Pods I believe the default selected was “Bluetooth Headset” which was causing this call audio to rout to the headphones, if they were not in the case.
This is a guess but I believe “Automatic” will keep the audio with the iPhone if it is held up to someones ear. If this persists I recommend running a “test” call between each other. During the call they would want to click the audio bubble from the Phone App. That will show the available audio options for the call and where the iPhone automatically routed the audio. They could then manually switch to a different audio option.
Apple has been pretty on top of using Machine Learning to let the products customize themselves to our natural preferences. After needing to manually correct the routing, a few times, I would expect the iPhone to then automatically start picking your spouse's preferred option.