If a user has deleted all their emails in Outlook, it is possible to recover those emails using the Office 365 Admin Console. Here are the steps you can follow:

1. Sign into the Office 365 Admin Console (https://admin.microsoft.com/) using your admin credentials.
2. Navigate to the Exchange admin center by clicking on the «Admin centers» option in the left navigation menu and selecting «Exchange».
3. In the Exchange admin center, click on the «recipients» tab in the left navigation menu.
4. Select the user whose emails you want to recover, and then click on the «Recover deleted items» from the drop-down menu.
5. A new window will appear, displaying all the deleted items for the selected user. Select the items you want to recover, and then click on the «Recover» button.
6. A confirmation message will appear, asking you to confirm that you want to recover the selected items. Click on «OK» to confirm.
7. The recovered items will be restored to the user’s mailbox.

Please note that there is a limited time frame during which you can recover deleted items in Office 365. By default, this is 30 days, but it can be adjusted by an administrator. If the deleted items have been purged from the mailbox, they cannot be recovered.