A healthcare workers' union is urging Boston's Beth Israel Deaconess Medical Center to investigate stockbroker Robert Jaffe, a member of the hospital's board of overseers who helped direct investor funds to accused Ponzi scheme mastermind Bernard L. Madoff. The union group, Local 1199 of the Service Employees International Union, United Healthcare Workers East, sent a letter to the hospital's board of directors urging it to "investigate Jaffe's conduct and remove him from the board of overseers if appropriate." Beth Israel had no comment on the letter.