He had a gash from throat to the end of his ribs it said. No, Tigerstar couldn't heal his wound, but the fatal wound from smaller injuries is usually healed when they are talking to StarClan. When Firestar fights Scourge, the blow to his head is healed with a scab left. When Bluestar dies of Greencough, it is healed when she comes back. I believe that they can heal some wounds, but not big massive ones like Tigerstar's gash to the throat.