You are correct if I remember the sequence. Essentially, she asks Evan to do that (after Evan says he didn’t write the letter), and then Zoe goes completely off on her brother, telling her parents that her brother was simply a bad person with no redeeming qualities. Evan sees how devastated they are, and in an attempt to lesson their pain, he starts to make things up and sings For Forever. His motives at that point were completely pure. Clearly, it spirals out of control, Evan begins to benefit from the lies as he achieves notoriety, and the writers do a good job creating moral ambiguity in the character, which according to what they said subsequently was intentional.