who's life does black beauty saves?

Black Beauty saves the master's life.

In Chapter 12, Black Beauty recounts the story of the stormy night. One night, John and the master had to travel a ways on business and they took Black Beauty as their carthorse. On their return journey a heavy storm was raining down on them. As they continued on their road, an oak tree came crashing down—torn up by its roots—right in front of the cart. To his credit, Black Beauty did not dash away or lose control, but only trembled a little. The tree blocked the main road so they had to take another, more dangerous road, which crossed over a large river. As they approached the bridge crossing over the river, Black Beauty hesitated. He did not want to go onto the bridge. His master nudged him on, gave him a little whip then a sharp cut but still Black Beauty did not move forward. Something was not right with the bridge and in a few moments they found out what that was: the bridge had collapsed and anyone who attempted to get on the bridge would fall into torrent below. The master, realizing his error, expressed great pleasure with Black Beauty. He pointed out that God gave humans reason but gave animals insight, and the one compliments the other. Furthermore, upon their return he praised Black Beauty as the reason why he and John and the whole cart had not sunk in the river.