There are several factors to consider as to when she should submit the FAFSA.
I'm going to assume that she is currently a dependent student - meaning that she currently has to provide parent information on the FAFSA.
If she applies after marriage, she would be an independent student and parent information is not required. But it will then requre her husband's income and assets. This could make a big change in her aid eligibility depending on your 2012 income and your assets at the time she files.
If she has received satisfactory financial aid in the past and nothing much has changed, she should expect similar aid by applying before the marriage.
If she applies after the marriage then it will only require income of the two of you - if you have a low income compared to her parents she might be eligible for more aid. But of course it doesn't mean that the school will give her more (unless she becomes Pell Grant eligible and wasn't before) and if she has already received school-provided grants she might be applying too late for them if she waits until May - check her deadline.
You can get an idea of what her Expected Family Contribution would be after marriage by completing a FAFSA4caster at the FAFSA homepage fafsa.ed.gov. The FAFSA4caster link is in the lower right corner. Then compare it to what she had for an EFC this past year using parent info, or even doing a 2nd 4caster with current parent info.
College Financial Aid Consultants
29 Ives Hill Court
Cheshire, CT 06410