Well, it's never a great idea to have lies between mates, but you already KNOW that, so I'll just answer your question.
It SOUNDS like you mean you think there will be traces of what you said on a website, other than your computer? I'm not sure.
If it's on YOUR computer, you can do what was said above, but if it's stuff already on another website, that's trickier. Is it something on a forum, cause if it is, there's usually an option to DELETE (like you can click DELETE this question, unless you select a best answer, then you can't delete it after that).. But if it's something else, you don't have as much options.
You can clear your browser history, but he may wonder. You can clear your hard drive (by backing up and wiping your HD), but again, he may wonder.
My advice is to come clean with him and apologize for going against his advice, and tell him you'll do your best to not do that again...and do your best not to go against his advice again.
He'll probably be ticked at what you did, but not as ticked if he finds out you did it, then tried to cover it up.
Think about it reversed. What if you advised him to NOT do something and you found out he not only did it, but lied about it and tried to cover it up?
kinda sucks......
So, FESS
BE HONEST
SUCK IT UP THAT YOU MESSED UP
and simply apologize to him
and try and do better with him
I learned the HARD WAY that it's best to just be honest with your mate when you mess up....
IMO
Best of luck to you and your situation.....
sucks about your friend though....she deserves to know.