A friend of mine will be attending an ABT performance of The Nutcracker at Kennedy Center and she is wondering what she should wear.
I've never been to the Kennedy Center myself but based on my own experiences attending ballets I suggested she wear dressy business attire.
When I saw the St. Petersburg Ballet there were some in the audience wearing some very very formal dresses though so I'm not sure if I'm correct in my suggestion.
What would you suggest she wear?

I was just in Detroit last weekend to see the Kirov, and I saw a large spectrum of dress. It was a matinee, but I saw satin opera coats, little girls in taffeta party dresses. I saw alot of suits, personally I wore one also. I think your suggestion would be appropriate.

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Something pretty and festive--a dressy suit with a sparkly pin, or long pants with a dressy fancy top, or frankly, whatever she wants and feels nice in. No jeans. Holidays are a good chance to dress up!

There will be lots of children in party clothes. If she parks under Kennedy Center, she won't have to be outside, so no worry about ruining fancy shoes. (Be advised, though, that parking is $15 )
There is a shuttle bus that runs to the Metro Station at Foggy Bottom, if she takes Metro and doesn't want to walk in bad weather. The Metro stop is quite close, however.

Unless this is some sort of highbrow gala, I agree that evening gowns and the like are unnecessary. Business wear (suits, blouses, jackets, nice shoes) with some nice accessories (scarf, pin, jewelry, purse, etc.) should work quite nicely.

If she, like me, prefers dressing down, slacks and a sweater should be fine. But if she sits in the expensive seats, she should look like she belongs there.

Also, matinees are more casual than evenings.

Unless the event is a gala and she plans to attend the dinner dance following the performance, formal wear is really, really overdoing it!

I go to the Kennedy Center quite regularly, and all of the above are right. I will note that the opera crowd seems to be more formal--I was there on a Sat night to exchange tickets and I saw all sorts of beautiful gowns and fancy attire. On weeknights many people will come directly from work, and on weekend (especially for Nut) there will be kids in party dresses. But really you'll see pretty much everything, so go for functional (depends on weather and your transportation arrangements) and festive.

PS Hey, I sit in the pretty-expensive seats and often do slacks and sweater, esp. on weeknights or for matinees.