I think that as long as there's no pattern of you playing with your friend, and that when you play him you both play properly (i.e., nothing that looks like chip dumping or "sandwiching" callers with reraises when one of you has no hand), you're probably fine.

And don't use the site's chat to talk to each other - even though the site may have no problem with it, best not to freak out people so they come here and start the 10,000th thread called "How much cheating is there?".

Send the site an e-mail to get their official word on it.