What do doctors do to treat torn cartilage in a hip?

Several options. If the cartilage involves the labrum, the labrum can be repaired, or cleaned up or debrided. If it is the cartilage covering the bone, this is a more difficult problem and may be treated by a technique called microfracture. The goal of microfracture is to create a "healing" type of cartilage to fill in the defect. These procedures are typically done arthroscopically.
Labral tear hip. Depending on the type of labral tear your surgeon might simply debride it or shave it so it doesn't tear further, or in some cases repair it back to the bone with a suture anchor. Articular cartilage is different and may require debridement, micro fracture or cartilage grafting from a cadaver source.
Preferrably repair. Torn cartilage probably referring to labrum which goes around edge of socket and forms seal around joint analogous to a gasket. Results of surgery have been shown to be better with repair of this structure vs simple resection. This can be done in an outpatient arthroscopic fashion with limited incisions.