... Everyone seems to think they have the "one and only one" approach. I don't know who SGOTI is, nor what you have against them. ...
True enough, everyone seems to think that, but when you look for statistical evidence and academic research to support their arguments, in all but one case, they fall away.
I am as greedy as the next guy. If someone shows me a better strategy backed by statistical evidence and academic research, I am on it. I don't think such a strategy exists, though, or the crowd that is fighting passive investing would have long since been beating us over the head with it. But I'm open.
SGOTI = Some Guy On The Internet
... Would you agree that "passive investing" is an approach with many differing details? ...
I guess that would be in the eye of the beholder. Nobel prize winner Eugene Fama says it very simply: "You have to hold the market portfolio." IOW, everything. To me, that's pretty simple.
... Would you agree that an investor needs to learn more than just the one term "passive investing" to be successful in their investment goals? Otherwise it sounds like the modern version of "plastics".
Well, not much more. I think that Fama would simply tell that investor to buy VT or VTWSX and be done with it. The only other question for some is whether the investor wants to have a degree of home country bias. That one is a little squishier because it adds the queston of exchange rates to the basic investing question. Fama's research collegue, Kenneth French, discusses home country bias in this short video:
https://famafrench.dimensional.com/videos/home-bias.aspx
(I am only talking equities here. I am not a bond guy but of course your investor will need to learn a little about asset allocation at least the effects of various weightings of equities and fixed income.)
Incidentally, I specifically use the word "passive" because the phrase "index investing" has been hijacked by the hucksters. They have been coming up with a plethora of mutual funds with "index" in the name and high fees, trying to convince naive investors that it is passive investing. Even the S&P 500 funds are not really passive. They are a sector bet on US large caps, IOW only about 40% off Fama's "market portfolio."