Well, obviously, this is not best practice... But have you checked the external sharing settings of the tenant ?
There are multiple external sharing levels, like on a list item, on a site collection and, lastly, on the tenant.
The external sharing settings are best described in the below link:
https://docs.microsoft.com/en-us/sharepoint/turn-external-sharing-on-or-off#change-the-organization-level-external-sharing-setting
You should also keep in mind that a user that has access to SharePoint, must be assigned a o365 license (even the lowest like F1).
I will try to stress out again that this is not best practice and external users are not meant to be tenant administrators.