I think your friend's friend is smoking something. I've never read in ANY Auto or factory service manual over the years any recommendations to run tires low on air pressure below the recommendation except maybe to get unstuck from sand. For one thing, you'll be inducing front end understeer which will make it handle poorer. Secondly, the added tire bulge will create increased temp at the sidewalls which stresses the tire's sidewalls.
Running tires low on air pressure was the KEY reason (but not the only one) that led to all those people getting hurt in the whole Explorer/Firestone tire fiasco. They recommended and sold Explorers with tire pressure of 26 psi ...when Firestone knew it was too low and recommended against anything less than I think it was ..30? (stressing my memory here) ..30 or 32. Even Ford engineers experienced handling and tire issues and recommended a higher pressure. The tire exploding problem started showing up in Saudi Arabia and middle east during really hot periods. Ford and Firestone sent engineers over to investigate the problem. They did nothing at first, then credited the tire cost. The problem was so persistent, the primary importing dealer would replace every set of Firestone's at their expense if need be prior to selling an Explorer. Then Ford started seeing tires explode in high numbers in the hottest southern states in the USA in the summer. In fact, a buddy of mine (now retired) worked for a large, well-known corporation and one of his colleges was killed in a company vehicle (new Explorer) on a mid-summer hot day ...the Explorer rolled over on a clear country road, no nearby traffic according to my buddy. The company lawyers attempted to sue Ford but Ford had assembled a swat team of lawyers for just this specific problem who would immediately fly to any Explorer tire issue, buy up the evidence ...and stay and watch it be crushed. The company my buddy worked for didn't win or settled out of court. Eventually the problem became so persistent (I think it was 1 out of every 350 Explorers would end up in a roll-over?? ...something along those lines) that it caught the attention of lawmakers and of course lawyers. Firestone issues were uncovered that exacerbated the problem it was shown. What came out of that big landmark lawsuit was mfgrs implementing TPMS to ensure tires would be run above a minimum pressure (so they wouldn't have to face lawsuits too), and SUV's being built lower to the ground so as to lower the roll-over risk. So if you ever wondered why TPMS came to be, and why SUV's are built lower ..well that's the story. BTW, this is chronicled in a really fascinating book that I couldn't hardly put down till I was finished, if anyone is so interested. It should almost be required reading for construction project managers and corporate engineers/high level managers of just about any industry:
https://www.amazon.com/Blood-Highways-Adam-L-Penenberg-ebook/dp/B00HR9B0A6/ref=sr_1_3?s=books&ie=UTF8&qid=1508168850&sr=1-3&keywords=adam+penenberg
I know this isn't an Explorer, they're not the Firestones and you're not at 26psi ...and if they get a little low for a period the world isn't likely to come crashing down. But there's a lot of info that running tires too low increases sidewall friction (temperature) between the rubber layers and if run long and hot enough it can lead to trouble best be avoided.
I would stick with the 40 and not take any more of your buddy's buddy's recommendations. Sure it might yield a little more compliant ride, but at what cost? These Rams already ride quite smooth as it is. Tires are expensive, treat them nice. If it's a brand new truck, they probably came nitrogen-filled which you'd be letting some of that out.
