In my opinion the best strategy would be to fully present that person the idea with all the ups and downs so he/she can have a wider picture of it. Considering that there is nothing ultimately bad or good we should have both sides of the situation. Then I would show portray to that person the situation in which his idea would come true and what would be wrong about it. With details and irrefutable reasons of course. After doing that I will explain him/her also what it would mean to not put the idea into practice or choose another variant and why it would be better to do that. If the idea would relate to my personal life I would also give some personal experience examples so I can make myself clearer.