Usually letters sent like this are firstly sent as a scare tactic. In the Hope you all cease with posts.
If Mr Howie decides to continue. Then each member whom he and his staff ridiculed via WhatsApp would possibly have a case against each staff member and TeamRH as a brand.
A company promoting positive mental health and positive body image but underlying find it acceptable to ridicule these members over a business chat page is not only bad for those staff members but also for the TeamRH brand. Especially if news media outlets pick up the story. In todays world anything considered bullying in the eyes of body positivity is a big no no and can attract a huge backlash.
Also Mr Howie, his staff and the TeamRH brand would also have to prove they didn't say what they said, provide proof of what was taken in the wrong context or manipulated. WhatsApp has the ability to archive its chat logs and each member has the option to archive or not. If the member sharing the chat has chosen to archive the TeamRH staff chat page then I can't see how those staff members can claim it was taken out of context or manipulated.
This could lead to TeamRH having to make a public apology and possibly a financial settlement to those members affected by this.