AFAIK and I am not a lawyer nor have been paying specific interest to this case, someone sending you images on WhatsApp is a crime, whether you download them or not.
From what I have read, the police searched his devices and they were clear - nothing downloaded. Just in the whatsapp chats, visible if you scrolled back. Yes he should have blocked the sender and reported the person to the police. But it's not the same as getting your camera out and making the pictures but the wording of the law makes it sound like that is what was going on.
Obtaining, holding, storing might be better terminology. I think in Scotland our legislation refers to "possessing an image" which makes a lot more sense, IMHO.