Sadly, the way Whatsapp works is that by viewing them, they're saved to your phone.Because he has saved them. If he had viewed them and instantly deleted them then no issues.
As I've said above, an iPhone not only saves them, but it saves them to your photo gallery, so they're mixed up with the 1,000s of other photos you have.
At least Android puts them in a separate folder.
The number of times I go through my pictures and go "Hang on, what's this one? where did this come from?"
Whatsapp downloads them even if you haven't stopped to look at them.
Can I point out btw, I'm not apologising for anyone. He deserves the full force of the law and technological ignorance is not an excuse.