Assuming your what thoughts on how to handle the situation and not on the specific wisdom of an intersection departure then;
1. If someone makes a mistake and knows they made a mistake, then you don't need to say anything. The scare that they gave themselves will be enough to change their future behaviour. Learning by making little mistakes is ok. This pilot in the 172 will hopefully remember this lesson when they are left seat in an A380 15 years from now.
2. If someone is not aware of what is happening around them enough to realise what they did was dangerous then a quiet chat from someone they respect is on order. A public chastisement will only lead to indignation and resentment.
3. If someone is too cocky to realise what they did was dangerous, only real options are official sanction or peer pressure.
The hard part is to pick the why so you know which approach to adopt.