Is The friars knowledge of herbs serves as a foreshadow?

Yes. Friar Laurence's knowledge of herbs foreshadows the events that will transpire in the play. For instance, he uses herbs to create a sleeping potion for Juliet, which sets in motion the tragic events that lead to her death. The friar's knowledge of herbs is also used to show his intelligence and cunning, which suggests that he may have ulterior motives in his actions.