Sounds like a valid concern about emotional issues; whatever your girlfriend thinks, her friend may have other ideas: how much and what it means, what it could lead to, etc. Sounds to me that, if going ahead with it is a serious possibility, all three of you need to have a sit down and establish what you're planning, what it means, what it will lead to, and what your limits are. Is it a one time thing? Only when your g/f is around? What does it imply outside of the bedroom?

Finally, take everyone's feelings into account but only do what you're willing to. If you feel it's risking your relationship and not worth it -- don't do it. On the other hand, understand that everything in life has risks, and embarking on something outside the normal scheme of things is going to be scary no matter how you do it.