Trying to work out how different people speak has been the basis of what linguists have been doing for decades. No doubt if red33410 ran a linguistics department his people would be walking into the Amazon to "present" various tribes with the "correct" versions of their languages.
Some examples from our language.
"I haven't done anything yet." (correct educated British dialect)
"I didn't do anything yet." (correct spoken American dialect)
"I ain't done nothing yet." (correct London working-class dialect)
All of these are part of the speech patterns of the various groups of people and mark people out as belonging to certain groups. If we are going to recommend one way of speaking to someone then it depends on the aims of that person. If someone from London or America wishes to sound educated, then we could recommend to them using in the British educated form. If Brad Pitt comes over to star in a Guy Ritchie film, then we would recommend the London form (by the way, there is nothing "stupid" about the double negative in the London example, many languages such as French, Russian, etc. have multiple negation marking and not because people in those nations are simply stupid). If we are in a high school in a rural area, then the standard dialects of English should be part of the curriculum so as to avoid exposing the students to later discrimination.
"He is better than us." is part of the most common dialects of British and American English (ie Standard American, British NRP etc.). If you use Latin-style case agreement in English then you will say "It is I.", "Was it they?" and so on which would make you a speaker of a minority-dialect - albeit one whose speakers often have a high education level. I suspect you don't though and are simply using those rules selectively in a few randomly chosen (by your high-school English teacher) situations and not in others.
What I suspect CatbertRules is asking, is "Leaving aside questions of correctness, why do you think it is unrealistic that the woman at the support group doesn't speak your minority dialect?"
Anyway, if analysis of speech and the aims of the student isn't the source of what we should recommend, then what other source of "correct" English exists? Some say "historical precedent", but I note that you are posting in 21st century English, not 19th century, so in your case this would be another example of selective application of rules.
People don't speak in grammatically correct terms the majority of the time, so to imply this poor guy with a broken leg should, is making me think you are trying to be better then us! :)