Run for the hills! Why would you even contemplate any idea of having an affair with your best friend's wife. This shouldn't even be a question and you should have formed the word "no" right off the bat. I get the feeling you are entertaining the thought of having an affair with her and if you do, you'll lose your dignity and any respect left by lying and cheating on your best friend. If he has you for a friend who needs enemies?
Marcy