Re: a solution
The article you linked doesn't say that WhatsApp spilled the message, but suggests that the UK police found some other means. Perhaps the recipient themselves got in touch with the police.
Not knowing the content encryption keys doesn't prevent WhatsApp knowing the source and destination of the content. In fact the only reason why Facebook bought WhatsApp was for this slurping opportunity.
The end-to-end encryption is bollocks anyway. Even with that, if you use WhatsApp to talk to,for example, a mistress, Facebook know when, how long, where you and she are, etc. And Facebook will advertise to you accordingly... So far as "damning information" goes, that meta data is plenty sufficient for most spouses to make their minds up, for law enforcement agencies to insist on you giving an explanation of your dealings with someone who turned out for be a drugs dealer / terrorist / burglar, etc.