In case the user is unable to access his account due to not having access to the Salesforce Authenticator app. It requires disconnecting the Authenticator from the user account to temporarily allow login without Multi Factor Authenticator. Kindly login using your admin account and follow the steps below to do this.
Once you are logged in in your admin account, please find the gear icon on top right to click and open the "Setup" page. Screenshot below.
2. In the quick find box, search for "Users" and select users as indicated in the screenshot below.
3. Select the name of the user "Fletcher" from the list of users. For example, "Alexander Robert" is selected in the screenshot below.
4. On the details page for the user, you will see a "Disconnect" button against "App Registration: Salesforce Authenticator". Click "Disconnect" and the device will be removed for the user. Once it is done Fletcher will be able to "Add Account" as he was able to do for the first time.
Note: Please make sure you remove the salesforce authenticator app from your old phone and then follow the above steps to remove the authenticator using the admin account