you said: I am wondering if I could simply post her tax information for the year 2012 and not that of the marriage, even though that is not how it was submitted.
--- NO, you can not & when you are asked to go through verification (complete with IRS transcript), you will have a serious problem & not be granted fin aid (including student loans) until you correct your FAFSA to match her 2012 tax filing
you said she is getting divorced, is she living separately? can she prove it? (for example legal separation forms... proof of different address from spouse) if so, then fill out FAFSA correctly, but talk with Fin Aid advisor about her being separated with pending divorce