Jaclyn,
Given the very short time frame of your trip, your choice will probably be influenced to some extent by your best choice (and cost) for flights. That will probably mean starting the trip in either London, Paris or Rome or other major airport.
Again due to the time frame, I'd suggest dropping Greece this time. Athens is good for about 2 days and travel to any of the islands is going to take time. Five days is just not enough time for that!
As some of the others suggested, Provence or Tuscany would be good choices. You'd have to allow travel time to reach Provence, unless you can find good flights into one of the airports in that area. Tuscany is very easy (and quick) to reach from Rome, and you could always fly home from Milan.
Another location you might consider for such a short visit would be Ireland, perhaps starting in Dublin and flying home from Shannon (depending on flights). Ireland has an interesting history and lots of character and of course "having a pint" is nice too. At the least, visiting Dublin & Dingle would be my choice.
In the U.K. you could have a look at either Bath or York, both wonderful places to visit, and of course Scotland or Wales. Lots of possibilities there!
If you decide on France, you might consider venturing into Normandy or Brittany (Bayeux, Honfleur, Giverny, Mt. St. Michel).
In Germany you might consider Bavaria and Munich. In addition to LOTS of things to see and do in Munich, you could day trip to Neuschwanstein and Fussen (an incredibly beautiful area!) and of course nice to "have a cool one" at the Hofbrauhaus!
In Austria you might have a look at Salzburg, Vienna or Hallstatt.
In Switzerland you could easily spend five days exploring the Berner Oberland.
There are endless possibilites for a short trip like that! You might want to check some of Rick's TV shows or Guidebooks to determine which area really appeals to you and your friend.
Good luck and happy travels