I can't say I have personally experienced a spouse and a BFF cheating. But, I was cheated on in the past, and I have to say, it was brutally painful. I felt stripped.
Healing From Infidelity: How do you handle it, when your spouse cheats with a close friend? Read about healing strategies.
I Cheated With My Best Friend's Husband (and I'd Never Been Happier). Deceit and guilt from being unfaithful to his wife consumed him. I began to find all sorts.