Trying to debate or argue probably won't go anywhere. The best I can offer is that you try and make her read a book. There are a ton of good books out there on this topic - I've written up a list here - and attack it from many angles.

However, I would actually recommend books not listed there, as they are recent publications and I made that list half a decade ago (!!!).

The two best options you could choose offer different approaches. You know your Mom best, so can pick which works better for her...

Option A is Robert E. Lee and Me: A Southerner's Reckoning with the Myth of the Lost Cause by Ty Seidule. Seidule is a retired military officer and historian who taught at West Point and first gained some national prominence for a short video he did for PragerU about how the war was about slavery. Might be worth also pointing her to that video, which is in itself notable as PragerU is fairly notorious for pushing a very conservative, right-wing slant in its history presentations, so the fact that they published a video like that matters!

In any case though I recommend Seidule because he offers an interweaving of history and his own, personal journey, being a southerner who was raised in the milieu of the Lost Cause, and hero worshipping ol'Bobby Lee. Its a really great book for someone who grew up in similar circumstances, I think, and its appeal is strongest in allowing them to confront the issue along with Seidule's recollections.

Option B is The Myth of the Lost Cause: Why the South Fought the Civil War and Why the North Won by Edward H. Bonekemper III. This book is just a straight history book, which provides a concise, to-the-point, play by play dismantling of some of the more common pillars of the Lost Cause. It is not the best book on this topic, to be clear, although it does a decent enough job.

The reason I like to single it out is a similar reason that Seidule's video is notable, in that Regnery Press published this book. They are an very conservative press (they published D'Souza's 2000 Mules, a "debunking" of the 1619 Project, and Ted Cruz's new book for instance) and their tagline is "Conservative Books for Independent Thinkers". I will avoid direct commentary on my thoughts there, but all the same, this strongly intersects with the typical person one would expect to buy into Lost Cause mythos, and being able to point to a book that they published and which does a no holds barred dismantling has a lot more credibility with some people than might an author like Blight, who can be written off, no doubt, as some liberal pushing critical race theory or whatever.

(A word of caution though... I straight up do not know how Bonekemper managed to get them to publish his book because they also published some straight up trash arguing pure Lost Cause bullshit by Samuel W. Mitcham, Jr., who isn't only an apologist for the Confederacy with books arguing, for instance, that NB Forrest - member of the KKK and commander of the Ft. Pillow Massacre - gets a bad rap, but also writes equally apologetic defenses for Nazi war crimes too. I mention this primarily to note that book is also available from the same publisher which... yeah. I don't get it....)

In any case, try to get your mom to read one of those. Or both. Or get her to listen to them as both are available as audiobooks. Its about the best you can try to do, in my opinion, and more productive than trying to argue it yourself.