My friend is experiencing chest pain or discomfort (quality: sharp or stabbing) (side: left) (severity: moderate) (time frame: sudden) and low back p?

Age matters: The most likely cause of simultaneous low back pain and left sided chest pain in a young woman would be an infection of your kidney or a pneumonia with pleurisy. Either of these should be checked out in person and treated. Very rarely, someone your age could have an issue with their heart that suddenly appears (at your age) or a problem with her pancreas ... so - take her to her doc.