Inorder to convince your wife to sleep with your friend, you have to
1- Explore her fantasies while you have sex with her. Once you get a clear picture, create some fake erotic stories and share it with your wife while having sex & keep doing it for few days because it'll be in her sub- concious
2- It will be much easier if she's drunk, so make her drunk on that day. Then introduce your friend to her.
3- Give your friend some private time to flirt with your wife & manipulate her when she's vulnerable.
4- Make it very clear that he's sexually attractive to her. She'll accept because she's vulnerable now.
5- Let him build up the sexual tension & turn her on with his touches.