A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
Hello Cornel,
To allow the external users to use Edit in Desktop app feature for the shared files, you need to give the access via Direct Access instead of sharing the file.
Open the Document Library> Right-click on the file> Manage Access> Direct Access with Edit permission.
Note: Please add the external users as a Guest Users into the organization.
Regards,
Chitrahaas