Revenge is most effective if it is very very subtle.
Since your friend probably already has some hearing damage, loud music isn't going to bother him. What you need is something soft and disturbing. Elevator music played just barely above hearing level is an insidious form of torture. But it's most likely to effect you as much him.
A change of tact is required. High power ultrasonic bursts are the way to go.
The tweeters are small and you can direct them at your target.
Nothing complex, just pick a frequency that is at the top of your hearing range.
Set it on a timing cycle to burst about once every 5 minutes for about 5 seconds between 6pm and 5am.
You victim should barely hear the bursts himself, but they will cause a kind of sleep apnoea and slowly ruin his life
You could try some secondary forms of attack, by torturing the parents and coinciding that torture with the punk's music. They might get the message. I built a TV jammer for a friend who used it to calm his aggressive neighbour. He soon got the message when his TV would drop out only during football games near goal. It also only effected one team ? hehehe
Something I haven't tried (yet) I haven't had cause
Is if he's on the same phase from the street, inject something like a 200Hz square wave into the phase.
Filtering in the amp may protect it.
I could suggest many other things.... no of them legal, but some more effective than others.