What is Sir Walter Scott's treatment of Jews in Ivanhoe?

In answering this question, you might wish to consider the historical context in which Scott wrote his fiction and to examine Ivanhoe in relation to other European literary works of the time (and earlier) in which Jewish characters appear.
A recurring theme in literature, which Scott employs in Ivanhoe, is that of a Jewish father-daughter relationship. Here it is somewhat reminiscent of Shylock and Jessica in The Merchant of Venice and Nathan and Recha in Gotthold Lessing's Nathan the Wise (1779).The latter, incidentally, takes place in the same historical period as Ivanhoe, the late twelfth century, though in Jerusalem instead of England. Scott's theme also foreshadows that of Eugene Scribe's La Juive (The Jewish Girl), the libretto Scribe wrote for Fromental Halevy's opera of the same name.
In Ivanhoe, Rebecca and her father, Isaac, are basically portrayed sympathetically. Isaac contrasts with the stereotyped (though partly sympathetic) portrayal of Shylock in The Merchant of Venice and even more with that of Barabbas in Marlowe's The Jew of Malta, though there is an echo of Shakespeare in the attraction between Rebecca and a Gentile man. Unlike Jessica in Shakespeare's play, Rebecca does not elope with a Christian and disappoint her father. One could see Scott's portrayal of Isaac and Rebecca as showing some degree, inevitably, of condescension in spite of his much more enlightened stance than that of earlier writers. Also, though Ivanhoe is Rebecca's champion and rescues her at the final tournament, Scott does not go so far as to have the two get married and ride off together into the sunset, so to speak. Perhaps, however, the absence of such a development shows a more enlightened approach than if there had been a pairing of the two, because Rebecca would then have had to do the inevitable conversion to Christianity, like other Jewish women in literature in that situation, even in the much more recent novel Freedom or Death (1953) by Nikos Kazantzakis.
As a final suggestion to understanding Scott's approach in its historical context, you might want to look up information about Rebecca Franks, a Philadelphia-born woman whose family were Loyalists during the War of Independence and who married a British officer and moved to England. It's possible that Scott had met her and modeled the Rebecca of Ivanhoe on her.