Can't Drag Corners

Hi

My boss uses Excel 2010 on his Apple Mac.

He has done something to the screen and can no longer drag out the corners on his document.
Please see attached picture.

He can double click on the top and this maximizes and minimizes but the screen isnt the correct size and he cant ajust it.

Any idea on how to show this please?

Thank you in advance.

How is he running this on his mac? Is the system dual booted? Is he running this over Remote Desktop/Terminal Services/Citrix or is he running a VM?