By providing the external users access to your SharePoint Online sites you can set up alerts for them and they can set up alerts with themselves , So Make sure that the External Sharing is configured properly as Office 365 Admin center . For more details check SharePoint External Users and Alerts
If the Alert ME
is still disabled , make sure that you have already shared this library to these external users .
If you need to set alert for a specific folder in your library, the alert me option is disabled , try the following
- Try to
View properties
for this folder.

- From the above ribbon click on
Alert me
and set the external users email.
