I just realized you said this went to a client mailing list which means you gave the information on the business page.
To be quite fair, this dietician is still using the guise of professional credentials to threaten someone with legals. And, they are already threatening the business by doing so. Now, this business doesn’t have a right to dispense nutritional recommendations to client as it’s not licensed but is still considered a professional business and it could be considered acting without proper licensure which, is where the disclaimer is a good idea. It’s the only place that this dietician has any standing room, just because the information was posted under the business instead of a personal page.
The disclaimer is good, removing the dietician from the mailing list is also good, and I’d probably also have sent an email informing them that a disclaimer had been added. As long as the post doesn’t tell anyone they could have any disease cured with keto and only talks about the benefits you achieved, you should hopefully be fine.
I have to consider that there are plenty of people giving diet information as a business but not as registered dieticians or even nutritionists but, they usually have a disclaimer.