The others may disagree with me here, but it is totally unacceptable for you to let her date both of you! She took vows against doing that crap. If you agree to it, then all she is doing is having her decision to CHEAT validated by her husband. "He agreed to an open marriage" Why don't you and your best friend just take turns with her in bed and let her decide? Heck, you could get his condom ready and hold his robe while he is with her. That is what she is asking!
Sorry if this is crude, but that is how I see the situation.
I know this is completely off the reservation, but I would have one last very serious talk with my "best friend". Just a reminder, the guy having sex with you wife is not your friend.
