Legal Question in Real Estate Law in California

It's not clear from your post whether the fence is "proposed" or already existing, and if already existing, how long it has been there.

I assume the two driveways are parallel and adjoining, and that the true property line runs between them.

The real problem in answering your question is that, in addition to rights to do pretty much whatever you want on your own land, there may be easement rights clouding the picture. Such rights may have been formally granted or reserved in the past, or may arise prescriptively, from long-term use. Your neighbor could conceivably sue you to "quiet title" to an easement, or to enforce easement rights. Or, she may have no rights.

In order for an attorney, on LawGuru or in person, to figure this out, the attorney would need accurate descriptions of the situation with more detail than given, and some time-lines to show the history of fence installation and driveway use going back at least five years. Also, the lawyer would need to know if any deeds or maps show an easement or otherwise describe the driveway setup.

A short version of what Mr. Whipple said is that you are assuming the actual legal property line is the line between the grass and the concrete, AND that the conduct of the owners of the two properties over the past five or more years has not created a right in the neighbor to access her car over the grass. Neither of those things can be safely assumed. So your question cannot be answered without doing the legal work to determine those facts. It is not possible to do that in an internet Q&A forum.