Apparently your friend tried to open a .txt file, chose Excel to open it with because maybe he didn't know what to choose, and didn't realize when he made that choice that there is a block that is checked by default that says something like "Use this to open all files of this type".
The shorter answer and the solution is this:
Highlight that file (you know what I mean; put the cursor on filename until it turns blue)
Hold the Shift key down and right click on mouse
Select "Open with..."
There is a block that opens up that allows you to choose what you want to open the file with. Make the selection and you might want to check that other selection that says something like "open all files of this type with this program"
It occurs to me after typing all this that u said .txt file. I don't rightly know what to open a .txt file with. If it is a Word document then use Winword and see how that works.
The difference between right-clicking on a document name andholding shift key down and right clicking is that holding the shift key gives you that "Open with..." option.