Be friends with her, make her think that u only wanna be friends. Hold good conversations with her. Get her to tell you everything thats going on in their relationship, their problems, whats lacking, and everything else. Use that information to your advantage. Say for instance, he dont spend a lot of time with her. So what u start doin is spending more time with her, takin her out and so on. Everywhere the man is lacking, you pick up his slack. She'll start fallin for u eventually. When she comes to you and tell u that he did somethin to piss her off, your reaction should be to make it seem as if he's treatin her like dirt, even if its somethin petty. Trust me, it'll work.