Hiding it is only making it more complicated and worse than actually telling the truth.
John, I definitely think you are doing the right thing by thinking about the situation critically, and knowing that something needs to be done soon.
There's no use telling her that he's only dating her to get back at you cuz by the sounds of it, you don't know that for sure. Soon enough he will probably just break up with your BFF. Sounds like you've got a big problem on your hands - especially now that your BFF and you are gonna see him a lot.
You're just assuming that he's still crazy about you, when he more than likely just got tired of chasing you around. Username: Anonymous Deal with it and somehow do something you know he likes right in front of him and that will make him want you more.
My advice is similar to Drew’s, just be honest with your best friend. That way both you and your girlfriend can be open about and enjoy your relationship without having to tip toe around.