No fee at all beyond 60 days, you (or them) get the full amount. They don't need a Mileage Plan account either. Either online or over the phone, cancel the tickets and ask for a credit certificate. Credit certificates and a PIN are emailed to you (or them) and you can deposit multiple certificates into your own Wallet account for use or just hold onto them and use them directly at check out for future flights.

https://www.alaskaair.com/booking/ss...servation.aspx

https://www.alaskaair.com/content/gi...-overview.aspx