My friends son was caught witha bag of canabis at age 15. 18 months ago. He was charged but not prsecuted.

It sounds like you are asking whether a offense committed in another country would should up in the criminal record of someone living in that country, which would be too hard for a lawyer here in the US to predict.  Your friend's son should be truthful on his visa application.  If the visa he is applying for requests information about the offense, he should be truthful about it, making moot the question of whether the offense would show up on his record since he may have to tell the consulate about it either way.