How do I open Outlook Express from my external drive on another computer?

While my wife's MBP is in the shop, she's using my MBA with her Seagate external HD. It's fully backed up. She has no problem opening Word on the Seagate, though to make changes in a doc she has to rename it. But when she tries to open Outlook Express from her Seagate, it tells her she can't open it because it's in backup. What does that mean? Shouldn't she be able to access her email from her external drive on my computer? And use it to send and receive as if she were on her own computer? Without it getting mixed up with my Outlook Express? Note: her MBP has Snow Leopard, my MBA has Mavericks.