Not sure I agree with you, since the claim he's being sued over is Epstein related. And, at some point there will be discovery. And you know that there are a bunch of photos showing Trump with Epstein and Maxwell, but also pictures of Melania. So, every time this lawsuit gets mentioned, it should/could be framed as Epstein allegations involving Melania Trump. Now, Fox will undoubtedly NOT say it that way...but, you never know.
Also, I think it is important to say something about libel laws. My experience is more Canadian focused, so this might not be the same in the US.
But, I don't believe quoting someone is protection from libel. I mean, Melania should have sued that biographer first, but she faces the same challenge she does with Hunter, which is she's a public figure, and courts have given a lot of leeway in regards to what is considered damaging for celebrities. That's one reason why the National Enquirer wasn't sued into oblivion back in the day for some of the things they said.
So, basically, I doubt that Melania would win this lawsuit, mostly because it probably wouldn't damage her reputation too much.