I don't understand why if the husband is a believer or not would be an issue. Our duty to God should not change based on our environment. Of course, this comes from my view that abuse is not a form of abandonment. That reeks of pop psychology, IMHO (got a useless degree in that field) :)
Scripture pretty clearly delineates a circumstantial difference with respect to whether or not the spouse is a believer (see 1 Cor. 7:15)