To be honest, I think you are jumping the gun a bit here. You haven't even called the girl yet. I think you should call her, and then see how things develop. You might not get to the stage of sex. If you do, then go with the flow. These things come pretty naturally, after all.
You will be better off telling the truth, no matter whet the outcome is. The point is that if you lie, and it is obvious, then you have got yourself into bed under false pretences. And that is a pretty nasty way to behave.
Oh, and your buddy is fooling himself.