She can file as if single and claim Head of Household if they have been separated for at least the last six months of 2010.
However, the couple should investigate whether it would be best to file JOINTLY, which is normally the best filing status tax-wise. Working together and using tax software (or a competent tax professional who can model the return several ways), they can determine which filing produces the best aggregate tax refund, then negotiate who gets how much.
Since last year, it is possible to split the tax refund between several bank accounts, for the separated couple could easily file jointly and have their share of the higher refund direct deposited into their respective bank accounts.
I have performed these services MANY times among my clientele who were getting divorced, so if you want MY professional help, email me at
[email protected].