GAH! help me please!!!?

My little brother cursed out my friend on AIM while i was away from keyboard (AFK) . i left without locking my computer because i was only going to be gone for a minute, and he has NEVER done anything this drastic. i apologized immediately and said that it was my brother, but now i dont think my friend will talk to me again >< crap wtf!!!!!!!!!!!!! he either thinks i hate him and i'm using the "oh, sorry, my little brother was at my computer while i was AFK" excuse just to curse at him, OR, he can't be sure who he's IMing when he IMs my screen name!!!! dammit!!! how do i apologize, and let him know that my brother won't do this again, and it WASN'T me!?