Hi, thanks for your message. They can easily take back their accounts if they have the email address OR phone number they had created the accounts with.

They should go to Forgotten password>Need more help>my account hacked.

The account will be locked until the code sent from the creator email address!

It's better they report it from a device they have used before.

There is no Instagram Hack! it's meaningless anymore... You can read this for more info https://instazood.com/blog/i-was-instagram-hacked-how-i-recovered-my-account/