A friend was just diagnosed with inflammation of the heart, is that a serious condition?

Myocarditis. This is called "myocarditis" about 30-40% of patients will be recovered completely with no complications, about 30% will have some cardiac damage (decreased function) and 30% will have serious complications including arrhythmia and heart muscle dysfunction.
Potentially yes. If caused by a virus, inflammation of the heart can lead to heart failure and serious life threatening arrhythmias. Immune diseases can also cause inflammation; the prognosis can vary depending on the individual and the severity. Even with viral infections good recovery is possible.