The influence of diet and nutrition on health is a very complex subject and widely misunderstood.
The first thing to understand is that there is no one diet which works for everyone. People have different biological needs and their bodies react in specific ways to different foods. Despite what is sometimes implied, there is no one dietary approach that works for everyone. Some people thrive on vegetarian diets while others experience ill effects. Some people do well with dairy products, others do quite poorly. Some people improve on high fat diets, others worsen. There are many ways to categorize and break down diet and different aspects of food, but there is no one universally beneficial category or diet.
The second thing to know is that there is no foolproof way for predicting which diet any given person needs for optimal health. There are various typing systems - such as the Blood Type Diet, Metabolic Typing, Ayurvedic systems, Traditional Chinese Medical approach, etc. - which can offer guidance in some cases. And there are a seemingly limitless number of "one size fits all" diets - such as veganism/vegetarianism, paleo, keto, low carb, South Beach, GAPS, low FODMAPS, etc. There are also laboratory tests for food allergies, genetics, etc - but these too are not completely reliable. In the end, the only way to know which diet works for you is to try it and see how you feel. There are ways to make this process easier but ultimately each person's experience and the way they feel in their body, is the final word.
The third subject to consider is the degree to which food/diet affects any given person's health. This also differs widely between individuals. Some people are profoundly affected by their dietary choices - with immediate improvement or worsening upon eating certain foods - while others can seemingly eat whatever they please with no ill effects (at least in the short term, though potentially even in the long term). A third category of people are affected by certain foods, but it is not immediately apparent - only by removing or adding certain foods for an extended period (e.g. weeks or months) can they discern the effects.
Considering all of this, Dr. Johnson focuses in his practice on the use of homeopathy to strengthen and balance each person's health. As people become more healthy and balanced, two things happen: 1) they become more aware of what foods work for them and 2) the body usually becomes less affected by dietary choices. In other words, through the use of homeopathic remedies which improve the functioning of the body, the process of finding and following the correct diet becomes a lot easier. Strictly following the "perfect" diet usually becomes less important. Food allergies usually reduce and sometimes disappear as the body improves.
Having said that, even when homeopathy is helping someone, diet still matters. At the very least, there are some general dietary principles that should be followed - and this comes down to the idea of eating "whole" foods versus overly processed or refined foods. In other words, whatever the given person's dietary needs, the choices should be made from within the realm of real/whole foods and not overly processed or artificial items such as refined carbohydrates/sweeteners, isolated proteins, juices, excessive deep fried foods, food dyes, etc.
Additionally, even when a person is functioning at their best they may still have a sensitivity or allergy to a food or have trouble digesting certain foods - e.g. gluten, dairy, beans, etc. This happens in a certain percentage of cases and seems not to be indicative of imbalanced health but rather genetics (or more properly, inherited biological traits) that cannot be overcome.
Also, there are certain types of medical conditions, such as auto-immune disease, where given the potential severity and long-term consequences of the disease, the person should be encouraged to keep their diet (and other aspects of their lifestyle) relatively strict,
Though diet and nutrition can certainly be important factors in disease, Dr. Johnson tends to focus in his practice on improving people's broader health through the use of homeopathic remedies. It is not that diet does not matter - it does. Dr. Johnson certainly makes dietary recommendations in a number of cases and he is happy to help guide you in dietary decisions. But he also wants his patients to be as strong, healthy and free in their lives as possible. The more healthy and balanced the body, the more adaptable it is and the less affected by diet (and other aspects of the environment).