Be honest, straight forward, empathetic and polite. Going into this you must know this will end up possibly heated, you will likely not be thanked but disliked but apologizing takes a big person to admit they did wrong. And if you believe your doing this to break them up for him to be yours chances are this will not happen as they both will be upset. Do what you need to do as it is not right he has been treating her in this manner as I am sure your not the only one. Learn from this mistake and don't repeat this in the future.