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."
Since when was the story that WhatsApp would not disclose the destination of the message? Apparently Amber Rudd was quoted as saying ‘this terrorist sent a WhatsApp message and it can’t be accessed’ which implies they were interested in the contents more than who received it (I guess in case they had then destroyed their phone, etc, if it was secret, but catching associates was probably easier).