So, I was writing a book with my best friend, and half way through - at 70,000 words - she decides she doesn't want to write the book anymore; but I still can. And we didn't have any agreements or anything because we - or I - thought that she wouldn't just quit... like we're best friends and put so much work into it. But since we first started I had decided I was going to publish... but now I'm not sure how I can. I have written 20,000 words since she quit (which was about a week or two ago) and at first she said that basically she should have her name on the novel too, if I get it published, because she wrote half of it with me... but I can't do that because she can't have the credit for the whole novel if I wrote one half by myself. So, after we had a big fight she said that I can just publish it... but I don't know if that means I can just have my name on it... and maybe just put hers in the acknowledgements? But then, I was wondering if this would make it more difficult for me to publish as there has been... well problems with ownership. And I am going to have two more novels going off this first book... so they would just be solely mine, right?
Any time you get into any business relationship, especially with a friend or relative, get everything in writing up front. And the more important the friendship is to you, the more important it is to hash out the details ahead of time. It isn't a lack of trust --it is a way to avoid hurtful misunderstandings.
With that said, I am not a lawyer, nor have I been in this situation, but if it were me, I would give her the co-writing credit and up to 50% of the royalties without feeling ill-used. 35k words is a not-insignificant contribution to a book, especially since 70k is a viable length for a novel by itself. Furthermore, no collaboration is ever actually 50/50, so an even split is always a convenient fiction to avoid hard feelings. Who is to say her contributions weren't decisive? Denying her a cowriter credit after that big of a contribution seems petty and unfair.
In exchange, I would want, in writing, all rights to the characters, setting and any sequels, as well as the unilateral right to make decisions (publisher, selling rights, etc.) for this book. That seems fair, since she has abandoned the project, and you haven't, and it should adequately compensate you for the extra work you will be putting in alone.
I'm not a lawyer, but it looks like you can't publish the book with the content developed by someone else without having some kind of agreement with that person.
If you rewrite 100% of that content, then the answer is maybe, but the other person can still sue you and win.
So my advice is to talk to your co-author and try working something out.
The others advising you sound pretty spot on she can technically sue. Or you could technically offer her a deal in writing (Which she can counter a deal to you as well) either way you will need a contract in writing maybe even notarize it.
If she wants future royalties what is it 50/50 or 60/40? She has to agree this is ok or counter and you agree, but also is she giving up rights to media opportunities, product sales, ect?
If you pay her for her labor and then she is willing to allow you to have full rights and wave her own over the project with or without a co-author or maybe even mentor status credit you need paper work for all of that.
Some ghost writers are included onto novels others are not you might want to see how they do that if you pay her for her labor she may fall into ghost writer status now and will she have a non disclosure agreement?
Contacting a lawyer would be best if you want to keep her contributions to the work because she still has intellectual property rights as well as you do also will this dissolvement of your writing partnership also mean she can't write her own world based on this material? Or do you only get to write this one novel with the co-created content and nothing else? You want the universe, lore, marketing, product, royalties, ect laid out as to what can and can't be done with it and by whom but you are also willing to have any lawsuits, complaints filed about the book targeted at you, and you are also letting her know you are responsible for fees for publishing, doing cons for promotions (whatever) and other things not her she is cleared of financially assisting in the products, websites, whatever.
A lawyer can help you figure out all the points if you miss something (or they do) that loop hole leaves you open if something good turns on your book for her to come back in with lawyers of her own for services or fees or royalties. You can't make her come back and she may never want to try to get any future monies or deals from this book but unfortunately its something that has to be contract specified now to avoid any misunderstandings or expectations down the line.