Why would my friend's 11 year old daughter, who has been hospitalized for three days, need a blood transfusion for pneumonia?

It could be more....: Pneumonia is unlikely the cause for her low blood count which may have required transfusion. But the cause of pneumonia(infection) may be in low blood counts due to disease in the bone marrow. So find out what her White cell blood counts are doing. I hope she doe snot have blood Cancer which can present as a serious infection like pneumonia.

Hard to say: You don't treat pneumonia with blood. However, if a kid's system is weakened by anemia, they are more susceptible to sickness (like getting pneumonia), and will not respond to treatment as well if you don't correct the anemia.She may have been borderline for a while and the illness led to tests that showed her need for blood.You want to fix all the issues.

Blood transfusion: Blood transfusion is given for inadequate blood volume (anemia), acute blood loss (think car crash), or inadequate oxygen carrying capacity. Not for pneumonia alone.