Doubling bitcoins is possible but its pretty hard, with websites its without any doubts not possible but that is already clear.
With trading there are some options but it also has some big risks to be honest.
You are right with trade it is risky, but it is the only way for to double our bitcoins, if he invest in some coins which will have the potential to increase much higher in value then with that he will see his bitcoins doubling up when the coin start to increase in value.