An East Boston teen who once helped save a man after he accidentally fell on T tracks was fatally shot Sunday over $550 worth of pot during a botched drug robbery inside an SUV, prosecutors say.Alleged triggerman Thorus O’Brien, 18, “popped up” from his hiding place and fired on 19-year-old Duncan Ketter inside an SUV when Ketter refused to hand over 3 ounces of marijuana he and a female friend were trying to sell for $550, according to prosecutor Catherine Ham.