I'm going to offer something from another approach here. I have often believed that friends are friends no matter what right? Can't she still talk to the guy? What's wrong with letting him know she thinks the gf is great for him and telling him that she's always care about him and maybe slipping the whole poly thing into the conversation somehow? I'm not talking about out and out telling him she wants a relationship with him but just suggesting the lifestyle and seeing what he thinks? People talk about sex with their friends all the time. If he thinks its a good thing then make the approach but with the truth thing, you know, no cheating on the gf, she has to know. I don't know, it could work.
There is no life without freedom, the freedom to be who and what you are.