It seems, from just reading what you've said, that this is much more his decision that it is hers... if they are both okay with it then that's great and credit to them for being able to do things like that (they must have an extremely strong relationship). Unfortunately, we are all humans and emotions and feelings will come into play at some point. If your friend is only doing this so that her OH doesn't go behind her back, then this is for all the wrong reasons. If he wants to cheat he will find away, this way he just has her consent even if she is not happy about it. She needs to seriously think wether she can think about her OH with another person and be fine with it. It's a very serious step to take and I can't imagine it being for the light hearted. I think you need to talk to her more about this and get her to think just how she feels about it and wether she really wouldn't mind or if she's just saying that so he won't sleep with someone else without her knowing about it.
Yes, as I have said previously, it was him that introduced it into the conversation. My friend would never have introduced the idea. She says she's ok with it, but then qualifies it by saying that she would rather it be like that than him doing it behind her back. That's why I don't think she's open to this idea for the right reason.
She says that they have an open relationship in regards to talking about this, but I feel there's a big difference between being open to talking about it and the idea of open relationships and the actual reality of it. I asked her how she would feel if she had to witness her boyfriend having sex with another girl and she replied that she didn't know. In my opinion, that's a big risk to take, agreeing to something that you don't know how you're going to react to it.
It's all well and good trying to justify it but she will still have to deal with her emotions and I worry that she won't cope.
I don't know what to do. It's got to the point where I don't want to talk to her about it, hoping she'll forget about the idea.
After our conversation last week I think I made an impression on her. She did say that she would tell her boyfriend what I had said so hopefully my opinion might at least make them think about what they're doing...I hope they re-consider.