Firefighter Robert Wiedmann, burned Monday over half his body while battling a Brooklyn blaze, will undergo the first of many surgeries Friday during what's expected to be months in the hospital and a year of rehabilitation to recover, one of his doctors said Monday. Wiedmann, 38, of Islip Terrace, will have the surgery at NewYork-Presbyterian Hospital/Weill Cornell Medical Center in Manhattan, where he is recovering, said Dr. Roger W. Yurt, burn-center director.