Reporting doesn't just mean devices being seized, neighbours curtains twitching, it means you're wife knows about your sleazy WhatsApp porn habit. It probably means your kids knowing. For someone high profile, it may mean the media finding out.
He should have blocked and not engaged in any further 'conversation' with the sender, but expecting someone to report it, knowing it would end their own marriage, career and risk prosecution, is something that many people wouldn't be willing to do. I understand that reluctance.
There should be a way of reporting these images being sent unsolicited, which doesnt require the recipient to wreck their own life in the process.