I think giving her photographic proof of you two shagging isn't going to help - it might make her see that he's a cheating loser, but it will also be forever etched into her mind. It's bad enough when you've got an online coversation or eMail being read over and over in your brain, but an actual visual of the act? That's not something you need to show anyone to prove it - that's just cruel.
I think you could tell her, but you'll probably come across a lying ex, determined to destroy their relationship... he'll be able to call you a psycho liar, and she'll probably believe him. Though, perhaps in the back of her mind she'll keep an eye out.
Another not-so-nasty way to tell her? Send her a link to his dating site profile (if it has a pic or link to his eMail address).