If she marries him, his first wife will leave him
Question

I'm a single mother.  There is a brother asked me to get married, but he already has a wife.  I said no for a while but, I prayed stakhara and I found myself saying yes to the brother.  There are a lot of gossip going on in the community about us talking.  People tell me there are a lot of single brothers out there why a married man?  Than , people say if you want to marry her than get married or leave her.  He's trying to make his wife understand that he does want to get married.  It might take time for her to understand even though she said that she will leave if he does get married.  I lost my best friend from all this fitnah.  I would feel guilty if I did marry him and his wife left because she wouldn't be able to handle it.  I might not handle it.  Allah(swt) knows best. Should I be patient and wait for the brother to try to solve things with his wife or should I just forget him and marry a single brother?
Praise be to Allah.The issue of marriage is a personal matter which concerns you alone. You are not responsible for whatever happens between him and his first wife. 
If you have several options, then choose that which is the best. But if you only have the choice between marrying him and remaining single, then I think that marrying him is better.