I have done a few threesomes and I can say from experience friends may look like a "safe choice" the reality is they can be quite risky. Has this friend previously done threesomes and have they approached her about having a threesome? If not, then question that must be asked is your girlfriend willing to risk loosing this friend? There is about an 85% chance this friend will say 'no.' Reason being most are not inot threesome and there is a good chance your girlfriend will loose her as a friend. However if she does not loose her as a friend chances are the friendship will become either strained or distant. Even if her friend has done a threeosme before there is no gaurantee that the friendship will return once the threeosme is over. My advice is if your girlfriend cannot risk loosing this friend then it is not a good idea.