Final answer:
In the Harry Potter and the Half-Blood Prince film, Severus Snape uses a potion, presumed to be dittany, to heal Draco Malfoy's wounds caused by the Sectumsempra spell.
Step-by-step explanation:
According to the Harry Potter and the Half-Blood Prince (HBP) film, the cure for injuries caused by the Sectumsempra spell is the potion dittany. In a critical scene, Severus Snape uses this potion to heal Draco Malfoy's wounds, which were inflicted by Harry Potter using the Sectumsempra spell. Although dittany is not explicitly mentioned in the film, it is shown that Snape performs a healing spell or uses a healing potion to counteract the effects of Sectumsempra. Dittany is more clearly identified in the books as an essential component in healing severe cuts and is often used in potions for treating such injuries.