She MUST use the online application form ,submit the application and pay for the visa. Then she applies for an appointment at VFS Global in Bangkok , pay again for processing and for biometrics, any copies or translations of documents are also charged for if not already done.
There’s a few got visas based on multiple appointments from a Thai dental clinic, these MUST be actual appointments and follow up treatments with dates from the clinic. As for the UK dentist but you haven’t got a chance.