I think that if you're on the MG route, you should help him if you're a good friend and you're loyal to your girlfriend. But I only play on "others" and I always fuck Camila on the dance floor, and it's unlikely that this will ever change. Therefore, I always tell Derek "no."
In my opinion, this scene is poorly written, and there's no option for me to respond as I want. I would say to him, "Dude, I like her too, so I won't help you." End of story. The answer I'm forced to choose makes it seem like I'm trying to help him get back together with Ashley. But that's not true, and I don't think Ashley is a good choice.