You know, there is a movie out (or was recently in theaters) with this very theme. It's called Chloe starring Julianne Moore and Liam Neeson. Its about a wife (Moore) who suspects the fidelity of her husband (Neeson) and sends a club dancer (Chloe) to try to seduce him. I didn't see. The preview actually unnerved me a little.
There is not an easy answer to this question. But I want to pose a theory about an underlying motive force that might be in play here. The girlfriend wants her man to cheat on her with this woman - so that she can watch. In other words, there is a voyeuristic impulse operative on the part of the woman.
By picking the woman herself, the girlfriend may secretly feel like she has more control over things. Sound preposterous? Consider this.
Suppose there is this televangelist who comes on every other Sunday preaching about the evils of bestiality - like he sees it as a priority to wipe the Earth clean of this particular sin. What if he has a secret fascination with it (even if never intends to physically do it or try it), railing against it all the time, allows him to keep thinking about it.
If I say to you "Don't think of a yellow elephant," - even if you tried to comply, you couldn't do it. That is because you have to mentally touch the idea in order to try to "put it out of your mind,"
Frankly, I would say the woman wants to either have threesomes or watch, but doesn't know how to broach the subject.