Well what is happening is the following:
In the tab area down at the right or the last tab you see a small tick inside the red circle like in the attached picture. Just drag it to the right to reveal the hidden sheets that were covered by the horizontal scroll bar. Save the workbook and re-open it, it should be fine.
gowflow
gowflow
This setting is regardless of the workbook and remembered by Excel so in your co-worker it must have been set too much to the left and reason why the sheets appear to be hidden.
gowflow
gowflow
ooops !!! Sorry my mistake. Your co-worker is opening Excel via web and not as an application like you are doing correct ???
gowflow
In the tab area down at the right or the last tab you see a small tick inside the red circle like in the attached picture. Just drag it to the right to reveal the hidden sheets that were covered by the horizontal scroll bar. Save the workbook and re-open it, it should be fine.
gowflow