She used to sit with him and his wife, and he decided to marry her and divorce his wife!
Mixing between men and women is haraam. See the answer to question no. 1200.
What you must do is repent to Allaah from what has happened of that, and resolve not to do it again in the future.
It is haraam for a woman to strive for her Muslim sister to be divorced, because the Prophet (peace and blessings of Allaah be upon him) said: “no woman should ask for her sister to be divorced so as to deprive her of what is rightfully hers and take it for herself.” Narrated by al-Bukhaari (2140) and Muslim (1413). So it is not permissible to encourage or tempt the husband to divorce his wife.
If the wish to marry came from the husband, and you did not try to separate him from his wife, it is permissible for you to accept, but we do not advise you to do that for several reasons:
(i)There is no guarantee that this man will not like someone else and choose her over you as he did with his first wife.
(ii)Your agreeing to marry him will stir up resentment, hatred and enmity in the heart of his wife, and you will not be safe from her ill-will and harm.
(iii)What you have mentioned about him working in the bank; he may leave it as he said he would, or he may not.
This is what we think. If, after thinking long and hard about the matter, you decide to agree to marry him, then pray istikhaarah and ask Allaah for guidance, and wait until he finds another job that is permissible.
We ask Allaah to help you to do that in which there is goodness, success and guidance.
And Allaah knows best.