Just let everyone know that he is jealous of you because you are bigger/stronger/better looking (whatever) and that he is just doing this to take you down a notch. Tell them you are just using creatine and amino acids.
When people have asked me in the past I got away with - "are you kidding, I fucking hate needles. Can you really see me sticking myself in the ass to shrink my balls?" Most just said "Oh yeah, I guess you're right" and left it at that.
As far as the former good bro - if he values his car like you value your reputation, hmmm (paint thinner all over the paint late in the evening to eat away the paint until morning, dead fish under his seat during a sunny day, leave a dime bag under his seat and call in an anonymous tip to the cops.)
Like others have said - It won't make it right but you will feel better.