Quote from Perfectionizt:

One of my friends , 17yrs old told me that sometimes she vomits a little bit of blood from time to time..does anyone know what this could be?

Has she taken NSAIDs regularly (Advil, Aleve, etc.)? Does she drink or have cirrhosis of the liver?

A little bit of blood in vomit is considered mild hematemesis, but it still needs medical attention. Massive hematemesis needs immediate attention.

Hematemesis is usually seen with Mallory-Weiss tears (esophageal tear), esophageal varices, peptic ulcer disease, and erosive gastritis. Get her to see a doctor.