What is the ruling on the marriage of a man who is already married, but cannot marry a second woman except by divorcing the first?

Your friend must cut off her relationship with this man and repent to Allah. If he wishes to marry her, he should propose to her guardian (wali). It is impermissible for her to ask him to divorce his wife, as the Prophet (peace be upon him) said: "No woman should ask for the divorce of her sister to empty what is in her vessel." If he divorces his first wife due to a law or custom and out of a desire to marry her, then there is no blame on her for accepting him.