Originally Posted by opalescent
It requires so much pain, and work, and dedication and willingness to listen and to really change that for most, it is not possible.
This pretty much describes, for me, what opening my marriage has been like WITHOUT any cheating or lies involved (beyond the ones I told myself and therefore my wife about being ok in monogamous relationship).
As far as the OP - I personally think your best bet is to just come clean to the gf about what you and her bf have been doing. Say your sorry and leave it at that. Don't try and stay in the picture right off the bat, let the bf do what he needs to do to save his 7 year relationship. Then, if the gf can manage to stay with him, and if she is a more forgiving person than most and is okay with her bf staying in touch with you (and he chooses to), then he can bring up the topic of poly and wanting to keep you in his life in a more romantic/sexual way. But only once he has rebuilt the damaged relationship.
Oh, another thing to consider is that in some cases, those that lie and cheat in a past relationship tend to be the type of person who is willing to lie and cheat in a future relationship (not always, and no judgement meant or implied on those who have genuinely made mistakes in their lives). Just something to think about.