Proceed with extreme caution!!!
It took some digging, but the following is from a somewhat similar thread:
Originally Posted by CDarklock
Then it's too late to open this subject.
Polyamory is like a business network. Your business network can help you get a job. It is better than anything else at getting you a job. And you have to build it... before you need a job. When you don't need one, and have no real expectation of needing one anytime soon.
Polyamory is good at getting you new partners and new friends and new relationships. It is better than anything else at doing it. And you have to build it BEFORE you need it.
Because once you need it, it is too late to build it. The attempt is tainted by self-interest, and will never be seen as honest. It's not impossible to build a polyamorous foundation under these circumstances... but it's very difficult, and calling it "doomed to failure" is really not all that much of an exaggeration.
Expect that for best results, you probably have to walk away from that crush. Your chances go way, way up if you do that.
You can check out the thread and decide how much it might apply to your situation...but it is somewhat similar...in that anything you and your friend might want to say or tell the gf will sound to be
in your interest...not hers.
I don't have any direct experience to offer on this subject, just my gut intuition, so you can take it or leave it for whatever that's worth.
You'd probably be best served staying out of the conversation initially. Some friends of mine are just in the process of getting divorced...and the wife was considering it for a long time, she only filed for seperation once she had a new guy to go to. Regardless of her intention, the optics sucked. Similarly if he starts the conversation with "I want to open our relationship to other lovers & be poly, starting with Qanda who is sitting at our kitchen table making pancakes for our kids..." how is that going to sound to her?
Now correct me if I'm wrong, but it sounds like the two of you have been carrying on behind her back for a year anyways...(if I read your use of "love affair" ) in which case there's probably going to be a lot of trust issues with her to overcome...because she's not likely to want to trust either you or him once that comes out.
Probably the most gentle way to ease into it would be for him to talk to his gf about the possibilities of open relationships or poly in general and see what she says. If she threatens to go Loraina Bobbit on his ass if he ever touched another woman...then don't hold your breath...it'll take a while.
If on the other hand she says it's always something she wanted to try...or that she didn't know it was possible but sounds interesting...then things will probably go a little smoother.
If he brings up the subject and she tells him "Sounds fantastic...I've been shagging a guy down at the office and I'd really like to get to know him above the deck instead of under it"...well then ...Game On!
I guess the point of all this, is that finding out her inclination towards poly would be a decent first step...
As for yourself...if you really want her to not feel threatened by your presence while this discussion is going on, probably best if you make yourself scarce. Probably a good idea to not being carrying on behind her back or there won't be any basis for trust to be built.
And be prepared to walk away...for your good, or theirs.... If she's not receptive and makes him choose....he may not choose you. Be ready for it...or more to the point, be ready to walk away so you can make your own choice.
Ok...anyone else with better insight...feel free to jump in and tell me to pound sand if I'm off target here.