be to Allaah.
There is nothing to stop you marrying him, whether he
divorces his wife or not. Your marriage to him would not be counted as
wrongdoing towards his first wife, because plural marriage is something that
is regarded as commendable according to sharee’ah for the one who is able to
treat his wives fairly. With regard to the problems between him and his
first wife, and his thinking of leaving her, you have nothing to do with
that, and you will not be sinning, so long as you do not ask him to divorce
her or encourage him to divorce her in any way.
But if he does not divorce her and he wants to marry you,
then he must treat you both fairly. If he fears that he will not be able to
treat you fairly then it is not permissible for him to engage in a plural
marriage, because Allaah says (interpretation of the meaning):
“… then marry (other) women of your choice, two or three,
or four; but if you fear that you shall not be able to deal justly (with
them), then only one or (slaves) that your right hands possess”