"Non-fiction novel" is an outdated term. It's what Truman Capote used to describe In Cold Blood--a novelistic treatment of an actual occurrence. The "non-fiction novel" was non-fiction, not fiction.
Writing a novel inspired by actual events, as Dave Eggers did in What Is the What, is something a bit different. What Is the What is fiction inspired by the stories told to Eggers by a child soldier.
In any case, I think that pitching your manuscript as "true crime" is exactly the right thing to do. As someone involved in a headline-making crime, you will have a lot more traction in the true crime market than in the fiction market.
On your friend's suggestion: It is very unusual, to say the least, for a copy editor to ask for a percentage of royalties. You're already looking at paying her something along the order of $2,400 up front--which is a reasonable but not remarkably low fee for the kind of editing your friend will be providing--so a royalty-sharing agreement seems like gouging to me.
If you feel you need an editor (and someone who plans to write one book about one headline-making event that happened to them is just the sort of person for whom it's worth hiring an editor--people who plan to write for a living should, as I say ad nauseam here, learn to edit themselves), I think you could shop around for a better price and/or someone with more directly related credentials and experience (i.e., someone with experience editing true crime books for major publishers).
I have edited for newspapers. I have edited books. It's a somewhat different skill set. Also, there are particular conventions and narrative approaches in the true crime market, with which people who have edited previous books in the field will be familiar and with which your friend will likely not be.