Never write something about someone unless you’re happy for that person to read it. This applies to WhatsApp, email, SMS, all forms of electronic communication.
The days of secret correspondence are gone. Not only is it possible you’ll one day be hacked, you’ll eventually send a message to the wrong distribution list.
Imagine accidentally sending a message criticising your biggest client to your biggest client. Difficult to recover from that.
Technology has made it so easy to communicate quickly, removing degrees of separation and exponentially increasing the risk of mistakes.
Rather don’t take the risk. If you have something nasty to say, don’t write it down.
Be careful. No more secrets.