Seal of approval for dentists and when too much sugar is harmful to teeth?

My brother grinds his teeth badly. He has sensitive teeth, and a filling has washed out over the decades (20 years). He's an investment banker and under constant stress. At a dentist appointment (he's retired, and his daughter took over the practice and has been practicing for a short time), he was told that the grinding wasn't causing any harm, and that wearing a retainer wouldn't harm your teeth, but that he was consuming too much sugar. To compensate, my brother does a lot of exercise (runs marathons, does CrossFit) and is careful with his diet—except for drinking three espressos a day with a teaspoon of sugar each, and not brushing his teeth after each one. Every now and then, he'll have a smoothie or a shot after exercise (yes, that adds fructose and additional acid), but he doesn't drink Coke, Fanta, or juice. The dentist told him to avoid sugar altogether, drink espresso without sugar, no smoothies, no shots, no juices, just water – as she supposedly did. It should be noted that the dentist herself has a figure like Ricarda Lang, and we wondered how someone with weight problems could give such lectures.

Then a tooth that merely had a mineralization defect needed a filling. The dentist interpreted it as severe caries. My brother has had this mineralization defect since childhood, and no doctor had ever wanted to put a filling in there. She also suggested that caries also came from sugar. My brother had caries as a child and hasn't had any problems for 20 years. The doctor said, "Yes, I should do something so you can continue living like this," and added, "You won't be able to give up sugar anyway," all in a didactic tone. Then my brother was supposed to write down everything he was drinking over the next few weeks and present it to her. I've never heard of anything like that before—for a 37-year-old adult. The dentist estimated the filling at €140, and my brother was supposed to sign it right away. My brother would pay for anything—if it made a difference.

This irritated me. How much sense does a sugar-free diet make in terms of dental health? A little taste and joie de vivre with small espressos is certainly not a deadly enemy for teeth. I think grinding your teeth actually puts more strain on them.

In general, what can you do if a dentist makes an incorrect diagnosis that ultimately causes harm (see mineralization errors). Is there an agency or some kind of "seal of approval" for good dentists?