i think your post accidentally shifted from "how would rules have helped the situation described by OP" to "what would a good DMG look like" right around here, and i'm not entirely sure that's the worst thing that could've happened to it.
Well, the idea (or perhaps the
hope) is that the two are synonymous as one approaches some limit (in the mathematical sense.) Rules that would have helped this situation being included in the DMG would make it a more effective, productive book. A more effective, productive DMG would provide the kinds of guidance and support that mitigate issues of this kind--not
eliminate them, since that's impossible, but forestall them before they take root, identify the ones that slip through before they can flower, and/or resolve the ones that slip through
that before they can poison the game.
As noted, nothing will be perfect. But we definitely can do better--if we work for it. Where "we" means both "the people who actually make the product D&D," "we DMs who run D&D," and "we general members of the D&D-loving community."
Mentorship would be part of that process. Good, supportive, guiding,
effective rules would be another part. Community support would be another--both in the sense of giving support
to the community, and the community working to support itself.
Rules don't make men virtuous, any more than handholds make infants walk. But good handholds, carefully placed and spaced, can certainly help us master the basic skills so we can start walking on our own. Rules work just the same for virtuous behavior, for whatever context that takes on. As I'm fond of quoting from Aristotle, "Some vices miss what is right because they are deficient, others because they are excessive, in feelings or in actions, but Virtue finds and chooses the mean." (Nichomachean Ethics.) By which Aristotle means,
for each situation, Virtue finds and chooses the point
between excess and deficiency that is appropriate, which may differ from one situation to the next. What is cowardice (deficient courage) on the battlefield may be foolhardiness (excessive courage) in the shopping mall.
And it turns out, rules are actually really useful for developing practical wisdom--
phonesis, the ability to make correct judgments--because they help us to learn what "excessive" and "deficient"
mean.