Questioner:

What is your advice for a sister who wants to marry a man who is already married?

 Shaykh Aḥmad ibn Yaḥyá al-Najmī:

May Allah ﷻ reward her with good! There is nothing impermissible involved in that,  rather it is upon her and all [other women] not to abstain from marrying a man who has one, two or three wives.