The green card is valid for only two years untill she gets a permanent resident card
And I think once they've been married for 3 years she can apply for actual citizenship and she won't need the sponsor anymore. But I think she has to prove that they have lived together for those 3 years.
Bitter_Cat, it sounds like there is more to the story. If her husband has been abusive toward her and if that's why she left him, she can get citizenship if she can prove it, such as through police reports.