Is working in banks unlawful (haram), and is it permissible for a girl to refuse to marry an employee who works in one?

It is not permissible to work in interest-based banks. The girl must refrain from marrying a young man who works in an interest-based bank until he repents and seeks a permissible source of income, because his money is impure. If he leaves his job at the bank, then there is no objection to her marrying him. Otherwise, he would not be a suitable match for her. The suitability considered in marriage is suitability in religion, and working with usury without necessity is detrimental to one's religiosity.