Yes, they can file an Amended return for a refund.
You have up to 3 years from the due date of the return to get a refund, so you can go back to 2014 if needed as that was due 4-15-2015, so the statue of limitations on getting a refund expires 4-15-2018.
Below is information on how to amend a return.
https://ttlc.intuit.com/replies/3288565
