None of you are poly...(at least not yet). And he needs a FWB? It sounds like YOU are at least interested in being poly, but that doesn't mean that they are. It sounds like she wants to take him for a test drive just to try him out, and that's 'swinging' to me or an 'open' relationship or maybe even cuckold, but not really poly.
Communicate, communicate, communicate. If you can't talk about this, then how many other things can you or have you not talked about. Do you love her enough to risk losing her and do you trust her enough to know you won't?