I think Sov has the right approach here. Approach him at least once more, but be calm and respectful, saying that this is just what you think, but it is ultimately up to him. Just tell him that you're watching his back, as a friend should. No matter what he decides, support his decision (at least publicly), and in the end, when she dumps him, you'll still be friends and he may listen to your input more in the future.