This is really very bad on his part. This is now a days in fashion. Children or students do so because now a days they are attracted towards junk food or canteen food rather than home made healthy tiffen. They bring their tiffens to schools but throw their lunch in the dustbin and then they consume junk food and parents think that their child had eaten his tiffen only. For solving the problem you should tell your friend the consequences of eating junk food on regular basis in a convincing manner, also you should explain the whole situation to your teachers by not telling the name of the student and requesting them to conduct some plays or skits in the assembly regarding this. Also strict rules should be implemented to solve the problem.