When a marriage goes through a crisis, it takes a lot of work from both people to get the relationship back on-track. Your wife has been betrayed by you and by her best friend. A 7-month affair is not a careless one-night stand. You didn''t mention if the best friend was also married, but if she is, she will have to decide if she wants to repair her own marriage or not.
There is no right answer to your question. You need to ask yourself WHY you are thinking of telling your wife. What kind of reaction do you think she''ll have? Affairs are deal-breakers for many people. She could be so hurt that she decides to divorce or she could be very upset but want to save the marriage. Unfortunately, what often happens is that the "lover" tells the betrayed spouse. Do you think her best friend might tell your wife what has been going on because you ended the affair? You need to consider all of these things.
The other issue is if you were exposed to any STDs and therefore exposing your wife. If the best friend found it easy to cheat WITH you, it''s very possible she was involved sexually with other men also. I strongly suggest you get yourself checked out. If everything is ok, then that will at least give you peace of mind about that issue.
I would strongly suggest marriage counseling. Explain to your wife that you''ve been struggling with some things, and that you really want to work to make your marriage better. Within the context of marriage counseling, the affair may or may not come up. You need to explore why you allowed the affair to happen in the first place. Most affairs are more about an emotional connection than a sexual one. Marriage counseling can help you both rediscover why you fell in love in the first place and help you learn to communicate your needs.
When we make big mistakes in life, the consequences can also be big. Affairs usually don''t stay secret for long. Be prepared for an extreme, emotional reaction from your wife. Be prepared for her to need some space and to be very angry at both you and her best friend. If you really do love her, focus on that in your conversations. Do whatever she needs you to do to regain her trust. I wish you the best.