All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Our advice to you is as follows:
First, beware of meeting this young man, staying with him in seclusion, or exchanging communication or the like; for such acts are the steps that the devil uses to lure people to obscenity and immorality. Second, if this young man is a Muslim with satisfactory religion and manners, you can accept his proposal. The fact that he is married is not a defect; on the contrary, a married man already has experience with women and thus, knows how to treat them. We do not comprehend the reason for your fear of ruining this man’s family. However, if you believe that marrying him will lead to a life full of disputes and problems for you, for him and for his other wife, then we advise you not to marry him and to forget him. Indeed, Allaah The Exalted will compensate you with a better husband.
But first and foremost, you should resort to Istikhaarah (guidance-seeking prayer) and consult those whom you trust to have a sound opinion from your family or others; indeed, "The person who prays Istikhaarah will not fail and the one who seeks consultation will not regret."
We ask Allaah The Exalted to choose for us and for you all that is good.
Allaah Knows best.