Hello,
It looks like your collaborator has not made any snapshots yet.
He can do so through the CHECK IN CHANGES button on the bottom right of the editor.
Afterwards, you’ll be able to pull his changes to your machine using the SYNC LATEST button on the bottom right hand side of the editor.
Changes are not passed back and forth ‘live’ - you need to repeatedly CHECK IN your work and SYNC your teammate’s work.
I advise to read up on best practices: Unreal Revision Control Best Practices