I don't typically do this but I could use some advice.

One of my best friends has a women that's been cheating on him. He caught her once before about six months ago and then last weekend he found out again. After the first time it was miserable being around them and there was very little trust. After this time I have no desire to even be around her. The problem is that he is still in love with her and does not see the road he is going down. How can I make this guy see that he is making a huge mistake?