Murmurs are abnormal heart sounds that are produced as a result of turbulent blood flow, which is sufficient to produce audible noise. This most commonly results from narrowing or leaking of valves or the presence of abnormal passages through which blood flows in or near the heart. Murmurs are not usually part of the normal cardiac physiology and thus warrant further investigations. However, they sometimes result from harmless flow characteristics of no clinical significance.
It's really difficult to tell the effects of steroids on his murmur because there are many different types of murmurs, some more significant than others. There are 6 different grades of intensity and pitch alone. Then 4 different configurations and the timing determines whether it's systolic or diastolic. Systolic murmurs start or end at S1 & end before or at S2. Diastolic start at or after S2 & ends before or at S1. These qualify different murmurs.
Now that you know a little about heart murmurs, let's see what kind of effects steroids could have on them.
Steroids affect the heart by increasing the risk of hypertrophy of the left ventricle much like Ubi said. This could progressively worsen a heart murmur. Depending on the murmur, this could either deteriorate health or not have a change at all. Another effect of steroid usage is an increase in blood pressure that over time can damage heart tissues & blood vessels. This can also worsen a heart murmur.
So basically the moral of the story is depending on the heart murmur your friend has is whether or not steroids will have an impactful negative affect on his health. Chances are if he's still walking around, exercising, & going on about his normal life activities, his murmur isn't serious. He should see a doctor before begining any AAS use to reevaluate his heart health.
