Larry should have sued his ex-wife!

Well at the time writers tended to only have one copy of their script - you know since they were typing them. Since the main character seems to be kind of anal, he probably didn't show his work to anyone but his wife. That way no one could say "yes or no" to the fact that he wrote it and his wife could take all the credit she wanted and no one would be the wiser.

That isn't as easy to do now that people mainly write on computers. There is always an electronic signature of some kind, plus writers (at least *this* writer) tends to keep back ups of everything in case of computer failure.

Another thought that maybe she edited the book so much she really did think it was hers and not her husbands work...

"God is Dead" Nietzsche
"Nietzsche is dead" God

If you believe what Owen's friend Larry says, Margaret is untalented and can't write.

It sounds like Larry had only one completed copy, and Margaret took it. In court, it would be his word against hers. Larry probably couldn't produce witnesses (e.g. friends, visitors, and his agent) who saw him writing a novel and actual completed chapters.