If the user is encountering an issue where they cannot access their account because push notifiactions do not work, they have remvoed their account from the authenticator, or their repeating authentication code does not work, you can re-add the account to the app with the following steps.
log into admin center
On the left pane, go down to "Identity," and select. This signs you in to Entra admin center.
In Entra Admin, go to users>all users> then click your user to enter your user's card.
In the column under the user's name, Click on "Authentication Methods"
Select "revoke multifactor authenitcation sessions" in the banner to the right of the user's name
let it run
Select "Require re-register multifactor authentication" in the same banner
Then, in an icognito window, have them sign in to portal.office.com. This will prompt them to re-scan the qr and register on the app.