One thing I must tell you op is that his passion and interest matters a lot because there are some people who don't have passion or interest but would want to venture in trading due to the profits. If you have fully explained everything to your friend then let him decides for himself whether to further continue the trading or not, so that when there is any lost then he would bear the consequences by himself. The best is to open up to him and roll along with him otherwise him might make silly mistake and still run back to you.