The sad part is that even when doctors such as your rheumotologist recommends a vegan diet and tells them of past successes, the people still don't listen! People have been brainwashed for so many years to believe that milk and other dairy products nourish our bodies . .. look at all the ridiculous commercials on TV! They pretty much pump it down their children's throats because they think it is so important for proper growth, development, and bone health. It's sick what the industry has done to people!
I agree, it is a challenge. I often just try to persuade them to consider a simple and short experiment. I commonly find that the elimination of dairy foods from my patients' diets has a profound effect on the course of their disorder. Other dietary factors also can have this effect but diary is the most common.
I suggest they try a short hiatus from all dairy - based foods. Often two weeks does the trick. I purposefully describe dairy as not bad enough itself but just not a good choice for them as an individual. I of course believe that no one should be consuming dairy but find that that approach is less convincing from the patient's perspective.
I described that the same qualities that suggest that dairy is highly nutritious also increases the likelihood that it would upset the balance of the function of the human immune system causing it to malfunction. Other foods have this capacity as well. They include soy, nuts, gluten and the vegetables of the night shade group.
When dairy foods are not implicated by this short experiment, I simply move on to another likely culprit, using trial and error until it is identified.
In short, attempting to convince the population as a whole that they were misinformed by their pediatricians, school teachers, nutritional professionals, parents and even grandparents is quite a challenge. Simply explaining that they as an individual may have developed an intolerance to any particular food or ingredient is more likely to have the desired result.