My friend's two day old baby has been diagnosed with a heart murmur. How common is that?

Relatively common. A heart murmur indicates turbulent blood flow, but does not necessarily mean that there is a structural abnormality of the heart (e.g. Congenital heart disease). In the first few days of life, many normal babies will have heart murmurs; the key point is that these infants need to be evaluated further to make sure that there is no indication of a heart problem.