I don't like my friend's new boyfriend and I don't want to participate in his birthday gift
What do you dislike about the boyfriend?
He drives around aimlessly in his car and my friend has started dressing like he's in highschool.
It's up to you who you spend your free time with. It's potentially harmful to spend time around people that you don't like because you may get into arguments with them.
He just made my friend worse.
If you feel this strongly, then stop hanging around these people. No one should feel obligated to hang with people who make them feel mad or feel miserable.