A crew was finishing setup of crane outriggers and then prepared to switch from a 3-sheave block to a headache ball/pill. An employee was helping to un-reeve the crane block. The operator swung the crane over to the side and placed the block on level ground. He got down from operator's cab and checked to ensure it would not topple over under its own weight. He then helped the employee remove pins from the block to remove the rope. The crane operator then went up onto the crane and heard the employee yelling. The block from the crane had shifted and pulled the wire rope down, causing the rope to tighten up. The employee's finger was caught between the crane boom tip sheave and wire rope. The crane operator ran to pick up the block and relieve tension on the wire rope. The employee suffered an amputation of the right ring fingertip.