I wish I had your problem. I too would be wracking my brain to solve it - by which I mean get my wife on board.Honest and straightforward with the wife might not be the best course. So I offer this suggestion. If your wife likes porn use it to your advantage, eg girl on girl, fmf, and the like. Also , it may kill the deal, but if the wife’s friend really is interested it will be worth the risk to gently bring it up with her. If she isn’t interested and blabs to your wife what a perverted pig you are, well blame it on my advice.If she is interested she will also try to motivate your wife as you two are now co-conspirators . At the very least you will be walking around with a boner just from talking with the friend.