Office for Mac and Office for Windows XP compatibility issues?

Trying to convince a family member to make the Mac switch. She uses Office for Windows XP at work but might use Office for Mac at home. Will there be an compatibiity issues with the programs? She is a heavy Excel user and takes a lot of work home. If she has to rearrange cells and fonts and such each and every, it would drive her (and the rest of us) insane.

Any advice would be most welcome.

She is looking into either a Macbook Air or Macbook Pro. Putting Windows on the Mac is not really an option for us.

Thanks Community!