It seems to be a Principal 2 issue.

The Financial Conduct Authority (FCA) has today fined Christopher Niehaus, a former investment banker, £37,198 for sharing client confidential information over WhatsApp.  The FCA found that Mr Niehaus failed to act with due skill, care and diligence

Not an apparently serious amount of info but WhatsApp also cannot be archived to Satisfy SYSC 9.1

The details of the information he shared included the identity of the client, the details relating to the client mandate and the fee Jefferies would charge for their involvement in the transaction.  Mr Niehaus also boasted about how he may be able to pay off his mortgage if one of the deals was successful. 

FCA Notice