what happened to Phineas ? Why did Gene lie?

The boys are involved with their "Super Suicide Society of the Summer Session." This involves crawling out to the end of a tree limb and jumping into the river. Gene and Finny decide to do jump off the tree together. Once they are on the tree, Gene "jostles" the limb, and Finny loses his balance and hits the ground shattering his leg. Gene lies for a lot of reasons. He lies to protect himself. He also lies to protect his relationship with Finny which he cherish so much.