My best friend's wife is ruining his life and I'm going to point it out to hi.
What are the reasons that make you think she is ruining his life?
Because he's never been this crazy until she showed up in his life.
How do you know she's the cause of his change in behavior? It could be something else altogether. It can be treacherous going when you involve yourself in others' relationships.
Maybe you are right but I'm doing this to make a difference.
You can involve yourself by checking in on your friend, but that doesn't mean you have to involve yourself in his relationship. Rather than point the finger at his wife without proof, support your friend and let him know you're there for him.