He didn't. He killed him.

Bluestar couldn't beileve her ears as her patrol came back into camp, fresh from the battle over Sunningrocks.

Tigerclaw. Killed. Oakheart. My former mate. From Riverclan. Bluestar stood on the Highrock, her whole clan facing her as they heard what happened. How Redtail died by the claws of Oakheart, how Tigerclaw killed Oakheart for killing Redtail. Bluestar glared at Tigerclaw for a second, then snapped out of it. He killed Oakheart. Her beloved. The father of her kits, that were now living in Riverclan, unaware that their mother was Thunderclan leader. Bluestar stared down at them one more time, then whipped around and stalked into her den, collasping into her nest. She curled up in a ball, pushing her grief away as sleep washed over her.