How does Jerzy assist Agnieszka and Sarkan in their quest to destroy a heart-tree in the novel, Uprooted?

Jerzy, a young husband and father who tries to save his cows even after they are corrupted by wolves from the Wood. After becominf infected himself, he provides a channel into the Wood for Agnieszka and Sarkan to destroy a heart-tree. Sadly, he dies during their efforts to remove the corruption from him while destroying the tree.