[Sheena looks up at him, her expression neither stern or judging, as he explains. Spirits like hers are connected to the planet by mana, and when it's gone, they're gone. And since she hasn't seen any type of spirit around, it's probably safe to believe a similar thing is true here:

Without a forest, there's no need for a Spirit to protect it, so it disappeared. But as she listens to his words and studies the stone in front of her, something else comes to mind.

Rising, she places her hand on his arm in a uncharacteristic gentle manner and looks up at him.]

You told me that people who didn't care who they hurt as long as they got what they wanted were the worst. And they are--if they don't regret what they've done. You regret it, so you're not like those others.

I think you're friend knew this and had faith in you, otherwise, he'd have never left you that message.