Exercise physiology is, at best, a nascent science. The problem isn't the science, it's the mindset of the types of people who are attracted to it and their lack of common sense and logical void in the interpretation and application of the subject matter. It is a discipline whose acolytes are continually trying to juxtapose the cart before the horse - or a more apt analogy - the crap before the horse.
Your wife's exercise physiology wannabe friend is simply making a common mistake that the idea that VO2max doesn't increase anymore after 20 minutes, therefore (logical fallacy), you shouldn't run more than 20 minutes. The problem with this nonsensical thinking is that VO2max is a fun factoid for some people, yet has very little to do with competitive running.
http://2008olympictrialsakatommyleonard.shutterfly.com/241
They do the same thing with intracellular responses to training, equating anything that increases mitochondrial density to being optimal for training for competitive running. Mitochondrial density and VO2max are measurable responses to athletic training, they are not the horse driving the cart. For most EP's, a little bit of knowledge about biochemical responses to training is the crap before the horse. They don't even have the decency to shovel the shit of the road.
A few years ago I was training with a friend at a gym. He had been training obsessively for a few months, I, of course, had not. We were on the stationary bikes. The tension on my bike was two notches above his. I was working at a higher cadence, and my heartrate was lower. I casually told him that I was 'in the zone' and that this was 'tempo pace'. He asked, 'how do you know?' I replied, "after 30 years of doing this you don't suppose I know what tempo pace feels like by now?" Then he went off on a tangent about mitochondria. I listened patiently to my friend, who has never taken any science classes above the general high school curriculum, until finally I asked him, "what's mitochondria?" He went silent for a moment and thought about what he was going to say, then finally said, "I see your point."