What does it mean when you have a dream someone else is throwing up red?

Dreams are about the dreamer. So the "someone else" you see in this dream represents something about yourself. Throwing up suggests the need to get rid of something from your life. "Red" might suggest a number of different things, from anger, to passion, to anything that is symbolized by the color red. Other details in the dream, such as the location or sounds or odd objects, might provide hints about what you need to "throw up" out of your life.