2 Step Verification or 2 Factor Authentication?
If 2 Factor Authentication, see if anything here helps. If necessary, she can use account recovery as explained in the link at bottom of the article.
Verification code - how to get one and sign in with two-factor authentication. *
If she can get a code and sign in, she needs to change/add the number on her Apple ID. Look in the Security section when signed in online. She may have to add the new number before she can remove the old number. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime on all devices/computers.
Apple ID - Manage