I admit that I can't remember a time where it was lowered for a foreign head of state.
President George W. Bush ordered flags to fly at half-staff at the passing of Pope John Paul II in 2005, President Bill Clinton did so for Yitzhak Rabin (1995) and King Hussein of Jordan (1999), and President Ronald Reagan honored Anwar Sadat in 1981, but the historical precedent most often cited is President Lyndon Johnsons bestowal of the honor in recognition of the passing of Winston Churchill in 1965.
Only a foreigner would lower the Stars & Stripes for a foreign leader.