Will I be committing a sin if I give a friend a game disc or buy him a story, even though I advise him to be careful not to waste his time, due to the possibility that I might be a cause for him wasting his time?

If game discs are free from prohibitions, then trading them is permissible. However, it is not allowed to enable someone who buys or borrows them if you are certain that they will be distracted by them from what Allah has obligated upon them. But if you are not certain, then advising them before giving them the disc removes any burden from you.