Complex sentence: Beginning in the late 1950s they took on as a client Carl Lindner, Jr., who was rapidly accumulating ice cream stores, supermarkets, real estate, and savings and loans, and soon Lindner essentially became Keating's sole client.
Split into simpler sentences: Beginning in the late 1950s they took on Carl Lindner, Jr. as a client. Lindner was rapidly accumulating ice cream stores, supermarkets, real estate, and savings and loans, and soon essentially became Keating's sole client.