I like the same guy, but I still want to help...?

So I've been talking to this girl, Ariel lately ever since she told me that she needed help with a guy. She said she really likes him, but because he's in my grade and she's one grade up at another school, she doesn't get to see him as much as I do. She made it pretty obvious who it was, so I asked. It turns out it's the same guy I like. I think I totally made a bad decision when I didn't tell her I liked him too, and now she wants me to be an advocate for her and talk to this guy to ask him what he thinks of her. I really, really want to stay friends with her, but how am I supposed to advocate her when I like him too? Should I tell her I like him, or what? Because I've been thinking of talking to him some more and maybe asking him out. Any help is appreciated!