A friend of mine whom I trust 100% states that a user he was exchanging messages via WhatsApp deleted the whole chat,on his phone as well. He was able to take a screenshot of the last messages before the chat had been cleared. I have seen, both the empty chat and the screenshot.The chat is totally empty,my friend's messages are also gone. Having read through WhatsApp's official FAQ I learned it is impossible to clear contents written by the receiver. As a software developer,I am sure, there is nothing impossible when it comes to hacking and it pretty much depends on the skills set, but in this case I would like to know how hard / easy would it be to perform such a hack by someone who is not a hacker? Maybe there is a software people download and use to hack WhatsApp account? I don't ask for recommendations of such a soft, just want to understand how accessible such tools are for normal users and how hard could it be to do this technically.
This doesn't really have much to do with "hacking". There is a command in the WhatsApp protocol where the sender can send to the receiver "Please delete message X". Whether or not the client honors that command is a different matter (it would be trivial for a client to simply ignore this command), but the official WhatsApp client by Facebook obviously does honor it.
So, there is nothing sinister going on, it's simply the WhatsApp client deleting a message in its own database because it was asked to do so by another WhatsApp client through perfectly normal usage of the WhatsApp protocol.
Note that there is a time limit built into the WhatsApp client, where after a certain time it will not delete the message anymore.
Also note that this will not wipe out the message from existence, e.g. it will still be present in any screenshots you made.