Further illustrating what a dildo this guy is. I assume that he means when you install a communication app, it immediately wants to add all your contacts to the app.
Security-wise, that in itself is a problem, but it still doesn't explain why an unknown party was added to the chat.
I get asked to use WhatsApp from time to time for international conference calls. It always wants to "suck in" all of my contacts and while I guess convenient if you intend to use it regularly, the idea of facing book having all my contacts doesn't sit well with me. Nor do I like the idea of having my number or contact information out in some "directory" somewhere. Hell, for that matter, I don't like facing book having my phone number.
So, if a client insists on using whatsapp, I'll install it temporarily, not let it "suck in" my contacts, and uninstall it as soon as I can.
Also, one time I failed to uninstall it, and I'm not sure what happened exactly, but a client of a Jamaican lawyer that sends me work started calling and texting on whatsapp directly. I assume they had my number and email from emails forwarded by the Jamaican lawyer, but they didn't ever call on my phone or email, just WhatsApp. I found that weird and unnerving.
/csb