She's a girl working out. Literally all she cares about is how many calories she's burned. Then she inputs those numbers into this app to compete with her girlfriends.
Well, she just has to double the number from the Watch before inputting them. That's not cheating, that's correcting
And I think who have the very reason why the calorimeter is never accurate.
The human body is pretty efficient. Imagine if the treadmill at the gym told you that running for half an hour actually burnt the equivalent of a fat free yogurt ? Most people would be discouraged - and that would mean less money for the gyms and less money for the treadmill manufacturers. For a business point of view, it's way better to tell the customer that, yes, they burnt at least a dozen cupcakes in the process...
"No way to know which of the two is more accurate right? Any scientific tests I could do to convince her?"
You could stick her into a calorimeter. Of course, you would have to need lots of money, lots of space and a non-claustrophobic wife. A calorimeter will be 100% accurate.
But really, the calorie display is mostly a gadget. There is no way it can be accurate, there are just too many things to factor in : age, weight, BMI, sex, running technique, shoes, training (a marathon runner will be much more efficient than your wife), time of the day, amount of sleep, illness, genetics...
It's just like scales that measure body fat, they can't be accurate and they are not.