What should I do about having given my friend a video game disc that contained a violation of creed, and he has not returned it to me, knowing that I have repented and he may have disposed of it?

If the penitent can remove the traces of his disobedience, that is better, in avoidance of the disagreement of those who made it obligatory. If you can command your friend to get rid of the disc, or retrieve it and dispose of it yourself, then do so. If you cannot do that, then there is no sin upon you.