There is no one-size-fits-all appropriate response to when you find your closest compadre has been shacking up with your ex. But, let's be real. Was she your ex, or was she a girl you occasionally poked? The gravity of the relationship plays a large part in how much anger you're reasonably entitled to.
Things to consider when formulating a response:
There is a difference between being blatantly malicious, and genuinely believing it might not be an issue on his end. Recognizing the difference between the two determines how appropriate your response is on the scale of ambivalent to legitimately livid.