In literature a foil can be defined as a character who exists to provide a contrast to another character, usually the protagonist, highlighting certain of the latter's personal qualities. Henry Clerval in Frankenstein definitely fits this description. Though a close friend of Victor's since childhood, he's almost his exact opposite in terms of his emotional life. We can see this illustrated in how the two men behave during their journey together. While Victor is gloomy, sullen, and depressed, Henry is seized with wondrous enthusiasm at all the marvelous sights they encounter. As Victor ruefully acknowledges with remarkable understatement:
The delight of Clerval was proportionally greater than mine.
While Victor is high-strung, agitated, and prone to nervous illness, Henry is a source of great strength and fortitude. He selflessly nurses Victor through the aftermath of his breakdown, providing emotional support to his stricken friend. Whatever happiness Victor is able to experience is largely down to the presence of his dearest friend.
The significance of Henry as a foil to Victor is that he provides a glimpse of what might have been, what kind of life would be possible without the hubris, arrogance, and intellectual vanity that Frankenstein displays throughout the story. So long as Henry's around, there's always the hope that Victor might have the courage to change and repent of his sins. But once Henry dies, then all such hope vanishes forever, and Frankenstein is left all alone in the world, his only remaining companion the hideous monster he has created.
In a sense, one can see Victor as representing science and Henry as representing the humanities. Victor is obsessed with new scientific and technological theories but pays little attention to the human costs of his activities. Henry, on the other hand, is mainly concerned with ethics.
Henry is more sensitive than Victor and aware of the needs and feelings of other people but less driven to act; his reluctance to impose his will and invade Victor's privacy contributes to the tragedies of the novel.
While Henry's great ambition is to do good and contribute to the community, Victor is selfish, focused mainly on succeeding in his own self-centered goals.
In terms of temperament, Henry is cheerful and sociable, while Victor is a melancholy loner.
Shelley sets up Henry as an ideal; he is handsome, sensitive, attuned to nature, and a great humanitarian. He appears almost too good to be true and is a somewhat one-dimensional character, functioning as little more than a foil and moral compass. His goodness makes his death and Victor's complicity in it more morally objectionable.
You're quite right. Victor and Henry are character foils. Here's how:
At school, Victor is the alazon, an impostor who thinks he is better than he really is. He rebels against traditional science and becomes self-obsessed with the dark arts. Henry is an eiron, a self-deprecator who is better than he appears to be. He humbly honors his professors and their fields of study.
In terms of Freudian psychoanalysis, Victor is the Id, the selfish one full of desire; Henry is the Superego, the conscience who reminds Victor to think of others, instead of himself. So says Enotes:
Victor's closest friend and companion, who balances his emotional and rational pursuits. He studies Oriental languages but passionately loves nature and life. Victor acknowledges that "[H]is wild and enthusiastic imagination was chastened by the sensibility of his heart." And unlike Victor, who wishes to learn "the secrets of heaven and earth," Clerval aspires "to become one among those whose names are recorded in story as the gallant and adventurous benefactors of our species."
In terms of knowledge, Victor is a man of science and Henry is a man of the arts. Victor is out to segregate himself from community: he works in grave yards and secret labs. Henry loves languages: his out to communicate with others.