Well, this seems to be one dilly of a pickle from your point of view.
Truth is much simpler this time. Be direct.
If you find yourself tongue tied and unable to come up with the right words, make yourself a script. Nothing much, just a cue card with your opening statements. If you feel yourself becoming giddy and start to joke about it, carry a safety pin and prick yourself. I find the best way to keep on subject is a little reminder that this is serious business.
And if that doesn't work, get angry at the friend for being in the way. Don't say anything about him being there, but use that anger to focus yourself. If the boy is truly interested, he'll tell his friend to get lost for a while.
Love potions? As far as I know, those don't really work. Boy, there was this one time... But I promised I wouldn't ever talk about that.