She needs to deregister iMessage I think. She can do that on her Mac laptop or online:

Deregister iMessage on your iPhone or online: https://support.apple.com/en-us/HT203042

Direct link for convenience (scroll down to “No longer have your iPhone?”): https://selfsolve.apple.com/deregister-imessage/

Edit: oh I see, the problem is with having the email registered with iMessage.

See “Change iMessage settings in Messages on Mac” [1] and scroll down to “You can be reached for messages at”:

Open the Message app on the laptop and click the checkbox to deregister the email.

Or she can change her Apple ID email address online [2] to some other one, which would deregister the email from iMessage.

[1]: https://support.apple.com/en-ca/guide/messages/icht39422/mac

[2]: https://appleid.apple.com/