I would always advise my friend that it is necessary to study all subjects in depth and perform well in the examinations to enable him to carry out higher studies in order to have a stable working career. But, at the same time, I'd advise him that merely obtaining good marks is not the barometer of gaining knowledge. Attaining knowledge on diverse things is most important for a human person.
My advice to my friend would be acquiring knowledge by reading books on diverse subjects, learning about various matters by watching television channels like National Geographic, History Channel, Travel & Living and so on. But, first of all, he should know most things about his own country. With the advent of the internet and satellite television, gathering knowledge has become easy; he should make use of these facilities.
Even for passing examinations, he must read and understand the various subjects and topics rather than learn by rote, simply memorising without understanding is not good.