This is where the problem of marrying for love and for convenience, clash. Your friend obviously has feelings for this man, but she needs to look at their relationship seriously. While the man may feel affection for her, it is plain for anyone to see that he is not in love with her, and neither is he in love with his first wife. History is repeating itself, he will cheat on your friend, like he cheated on his first wife with your friend. It is the pattern of a man who knows he can have more than one wife, but is not in love with any. Your friend will have to accept that the man will never be in love with her, like his first wife had to. She must also accept that the man will most likely take more partners as he searches for something that is missing in his current marriages. That my friends, is the endless search for love.
As his second wife, she should have been prepared for this, I find her a little naive, but she loves him, so her mistake was that of blind love. If she truly wants a partner that loves her as much as she loves him, then she will have to be strong and leave him, but I fear she wont be able to.