"Telling someone that something is impossible" is a good habit to unlearn. It's easy to find reasons why something cannot work; much harder to find ways in which it *can*, and insisting that it cannot will do nothing but demotivate people and sap away energy that could have been spent on finding solutions you weren't aware of.

There's one exception: if you are very experienced in a topic, and you feel that someone is underestimating the difficulty of something (especially if it's common for people to do that); it can be worth warning someone about it. But if they indicate that they understand this, you need to take a step back.