Complex sentence: A German-Jewish musician in Gwendolyn Harleth's social circle, he marries a wealthy girl whom Gwendolyn is friendly with, and also advises Gwendolen not to try for a life on the stage.
Split into simpler sentences: A German-Jewish musician in Gwendolyn Harleth's social circle, he marries a wealthy girl with whom Gwendolyn is friendly. He also advises Gwendolen not to try for a life on the stage.