So, if this friend is deeply offended or creeped out by your overtures if she's not interested, would your wife be ok with losing a friend? Personally, I think there are way too many other outlets to meet people to play with, both in "swinger world" and "vanilla world" that I wouldn't jeopordize a friendship, but if you feel this is the best way to procede, getting your wife to facilitate a conversation about experimenting with women and whether the friend had done it or thought about it.
Something else to consider: since you've played "footsie" with her under the table, and she didn't object, maybe she thinks you're just interested in cheating with you behind your wife's back. A lot of people don't understand swinging, and are freaked out by it, but cheating is considered "normal". In which case, it's reason 359 why I really wouldn't try to convert a friend.
Tread lightly....I could see this turning into a powder keg....