Well, one thing's for certain, the old philosophy hasn't changed here by using new labels (poly etc). The risk is STILL there to affect the friendships just because if things turn ugly your hubbys friend may be forced into a position that he has to 'choose' - wife or best friend. So it's good to keep that on the horizon and decide if the risk is worth the potential gain (possible relationship).
Now.........if it were me, I'd also want to get more info direct from the horse's mouth (her). You might arrange a meeting with her on any acceptable need (shopping, lunch etc) and just open with........
"Oh - hubby tells me you guys decided to open your marriage too !. We've been open for years and find it helps our own relationship a lot ! "
From there you could ask about what THEIR guidelines are (so far) such as are existing friends excluded because of possible complications. If she's not sure (haven't come to that topic yet) you can use yourself as an 'example' - i.e. "what if your hubby wanted to hook up with me ???"
You should be able to get a lot of feedback from such a conversation. Things can proceed (or not) accordingly.