Does the marriage of a divorced woman to a married man involve injustice to his first wife, and does it involve injustice to herself?

It is permissible for a man to marry a second wife, provided he is just. The consent or knowledge of the first wife is not a condition. The second wife is not considered unjust to herself or to the first wife, because she has done what the Sharia has permitted. It is advisable to investigate the man and ask trustworthy people about his religion and character, and then perform Istikhara.