Looks like you didn't understand....
So you're basically saying ' it's okay for innocent girls getting taken advantage of (exploited or abused) by men with less noble intentions and whatever their intentions might be so that these girls could learn from it '. Maybe that's not what you meant, but that's how it is. You can't possibly predict what might happen to her or what might not. No offense or anything, your opinion sounded shallow to me. Not to mention, it's an extreme measure on your part.
Letting Zak have his way with her is an asshole thing to do when you already know about his true intentions and especially when he can be stopped. Emma will learn about his intentions eventually and will be heartbroken in the end either way.
I've seen a lot more innocent teenage girls than Emma, which is normal. Generally the age between 18 and 21, they are given rights and responsibilities of an actual adult. This is also said to be the age when their innocence is lost. So It won't be long before she becomes a more mature young lady.
Usually, this happens if the girl was so sheltered, she never dealt with men with less than honorable intentions before.
Which in Emma's case, is just about right.
Whether Zak goes all the way or not, is to the writer's discretion, but, Emma does need a real life kick in the teeth soon, or Zak is more than likely to have his way with her.
Let's be blunt here, most girl's first time, are taken by someone about their age, that only had one thing in their mind, and it wasn't love. Some are lucky, and some love was involved, others, were just used, fucked and dumped.
Emma being sheltered this much for so long, is not aware of this, or at least, doesn't believe godly Zak is that way, despite seeing him fuck her friend (if I'm not mistaken)...
I mean, if she still goes through it despite seeing that, well, she'll be partly to blame in my books for allowing it to happen. The being naive excuse can only go so far... If you see a guy fucking your friend, and never actually dating her later, then getting interested in you, you really don't need to be a genius to see there's a potential problem cuming soon.
But the bigger blame, will be on her mother's over protectiveness.
But on a more personal side, as shallow as it is, yes, I don't think being innocent or stupid is a valid defense if someone takes advantage of you. As long as there is consent (and both parties are of legal consent age), as far as I am concerned, there is no issue. That is the very definition of age of legal consent. Otherwise, innocent and stupid people, would literally never get fucked.