It's scientifically proven that weed causes way less physical and psychological harm than alcohol. However there are enough examples on how weed can destroy someones life. On the other hand, there are probably even more examples of lives destroyed by alcohol.
So yeah, if alcohol is legal, why shouldn't be weed as well.
But everyone is aware on how weed is illegal. It's everyone's own decision if taking weed is worth the risk or not. Your friend has made his decision and even went so far as to sell it. So it's his own fault, not the laws. However I do hope for him that being expelled from school is the worst thing to happen for him for that. Police records can be rather bad for your future life.