There are no doctors on this forum and it sounds as if your cousin seriously needs to see her GP or liver specialist. The reason for this one symptom could be multiple and varied.
This is info from MedLine, an on line encyclopedia:
It can sometimes be difficult to tell the difference between vomiting blood and coughing up blood (from the lung) or a nosebleed.
Conditions that cause vomiting blood can also cause blood in the stool.
Causes
The upper GI tract includes the stomach, mouth, throat, esophagus, and the first part of the small intestine. Blood that is vomited may come from any one of these places.
For example, vomiting that is very forceful or continues for a very long time may cause a tear in the small blood vessels of the throat or the esophagus, producing streaks of blood in the vomit.
☞ Swollen veins in the walls of the lower part of the esophagus, and sometimes the stomach, may begin to bleed. These veins are present in people with severe liver damage.
Other causes may include:
★ Bleeding ulcer in the stomach, first part of the small intestine, or esophagus
★ Defects in the blood vessels of the GI tract
★ Swelling, irritation, or inflammation of the esophagus lining (esophagitis) or the stomach lining (gastritis)
★ Swallowing blood (for example, after a nosebleed)
★Tumors of the stomach or esophagus
http://www.nlm.nih.gov/medlineplus/ency/article/003118.htm
Best of luck to your cousin
Probably a stomach ulcer. I suggest that she goes and see her doctor