Divorce and Marriage Timing - any advise will be appreciated

[djmikmik]

Me and my girlfriend are planning to get married. The problem: she is living separately from her husband (in the past 6 months) and would like to get divorce so we can get married.
She has a green-card and has an INS appointment to provide her fingerprints in 2 weeks in order to get her citizenship.
I am on H1-b Visa and started the application for a green card 6 months ago (based on my company's sponsorship).

We are not sure when is the best time for her to get divorced. Should it be after the fingerprints stage? only when she gets her US citizenship? is there any linkage between the citizenship process and the marriage process?

Appreciate any input from you.

D.