You can't. Your best bet is to discuss how wonderful your boyfriend is even if you have to lie through your teeth.
tell him you're gay.
oh. well that's out... if he is your friend, you could tell him that the friendship is too important to risk in a possible bad break-up. if he is not your friend, just tell him to "kick rocks, ***-hat!" THAT usually comes through clearly.
Dose? I wouldn't be using rohypnol. You'll end up in jail.
Tell him you are not interested, and set him up with your pretty friend.
That is very sad. She would have made him forget you.