Wow that sucks man. What i would do in this case is if i'm ask to perform for a group of people and the heckling friend is within that group, i would simply say "Well i'd love to show you some magic, but ____ here will probably ruin it, and that's no fun is it? Maybe another time" and then walk away. That way, the people who really wanted to see magic will take care of the friend themselves. If that friend isn't in the group, then you can go perform for them.
Actually he said one of his "friends"...
That makes a lot of difference.