User seeing things different in Excel..

Does this user have the same version of Excel as everyone else?

Does the file have any external references?

Does the file have any macros?

If they are opening it directly from the email, I would suggest they instead save it, then open from their local disk.

To elaborate, what functions are they not seeing?

There are 2 likely answers to this issue:

1. They are not using an Office 365 version of Excel and others are, thus they do not have some newer functions only available to subscribers.
2. The functions are not built in functions and come from an addin or VBA

Testing the 2nd is somewhat easy. Open Excel in safe mode on a machine this file works in and see if those functions still work. Any functions that come from an addin (either 3rd party or created internally) will not run and the functions will fail. Not sure that this will prevent macros from running however, unless they are in a startup folder in which case they wont open in safemode.

The first is as easy as going to File | Account and checking the product info.

Non-Office 365 may say something like:
"Product Activated
Microsoft Office Professional Plus 2013"

Office 365 will say:
"Subscription Product
Microsoft Office 365 ProPlus"

Another way to check for macros, is to look at the file extension. .xlsx will NOT have any VBA, .xlsm MAY have some code

Also, can you tell us what version they are all running?
(and maybe a stupid question, but how did you check?)