@unk NO wn,
It's unfair for him to ask you for advice on such a topic, seeing as he knows you like him.
What are you supposed to say? Oh, go back to her? Or oh, come to you? Either you deny your feelings or you give into an obvious conflict of interest.
If he was a good friend, as good a friend as you claim, he would not be putting you into such an untenable position.
It's unfair. It's unkind. It's selfish, too, and may even be cruel.
Tell him to cut that **** out, too. You don't deserve to be toyed with like this.