I'm having a small issue with one of my agents can't see the helpdesk tab, I tried to add it as a new tab, but that's not possible. He's the only agent that can't see it, all the other agents can.
If your colleague doesn't have the Helpdesk tab he may have deleted it, the tabs indicated in your screenshot are the default tabs, but they're not global and can be edited/deleted. The default Helpdesk tab should be visible to all active agents.
You may need to manually recreate the tab on the user's account, all of the widgets available on the default Helpdesk tab can be dragged in manually.