It depends on the vesting requirements. Unvested stocks are given to employees and have requirements such as job performance and/or length of service. Once an employee reaches the requirements, the stock becomes vested. It seems that the leaker didn't reach the goals, so the stocks are still unvested.

Page 11 of the complaint makes it clear these are vested shares that had as part of the requirement for receiving them that he maintain confidentiality. So they are claiming breach of contract and want the money back.

https://www.documentcloud.org/documents/24522887-apple-v-and...