I don't know your exact circumstances, but it might be worth filing for her anyway because of the making work pay program the IRS have this year. Basically, if you make between the minimum and the maximum amounts (can't say what they are off the top of my head), you qualify for $400. I was sceptical at first, but your post reminded me about my tax filing. I just checked my US account, and the $400 from Uncle Sam has been in there since the end of April. It doesn't matter where the income was earned, as mine was all earned in the UK.
Personally, I think the IRS should stop making me file every year as I don't live in the U.S. anymore, and probably never will. So, they could have saved themselves $400 plus administrative costs by behaving like every other civilized country and tax on where you were resident for the year rather than what country you are a citizen of, but I digress.
Sorry, that doesn't really answer your question, but I've never turned down $400 for nothing in my life and I'm not about to start now!
-Sam