My dentist literally took a photo of my x-ray with his phone and sent it to to my orthodontist via Whatsapp and everything went quick and smooth, much faster than the official channels. Solutions to get a job done quickly and efficiently in case of emergency always exist, they're just not "by the book".
Hippa doesn't apply in Europe but GDPR, and I don't see how that would be in violation since my information was exchanged only between the two parties with my consent, on an encrypted channel.
They would only get into trouble if that info would leak in an identifiable way to unauthorized third parties and would cause damages (here there's no punitive damages like in the US). And people here tend to guard their WhatsApp chats pretty well since it's what everyone uses and it also contains their private chats so in a sense it can even be more secure than the official medical channels which are just more burocratic but offer no actual guarantee of more data security.
Say WhatsApp is found to have a security hole that has been leaking data to 3rd parties. What may be the fate of dentists / doctors that decided to use it an "encrypted channel" for medical records? Are doctors / dentists not fat targets for lawsuits? What might the guidance be from their lawsuit insurance policy?