MDI (Multi Document Interface)
In Excel: can MDI be turned on and off?
If so, How?

Otherwise does anybody know how to reactivate it?

A co-worker opens several spreadsheet and each of them show it's own button in the task bar.

On mine it only shows one. I can simply go to Excel menu bar , Window and select which sheet I want to view.

She can do this too but the items in the taskbar are taking too much real estate there.