You have touched on a broad range of subjects having to do with heart disease. A person that needs a bypass has coronary artery disease (CAD) or blockages within his/her coronary arteries. This does not necessarily mean that this person has cardiomyopathy.
It can lead to cardiomyopathy, if a person has a heart attack that damages a large portion of the heart, or has chronic CAD over a period of time and somehow does not have a heart attack. When the coronary arteries are blocked the heart muscle does not receive enough blood through these blocked arteries. Usually this causes a heart attack.
Cardiologist can stent a patient enough to keep him/her alive, but still their heart changes due to areas of the heart muscle not receiving adequate blood supply. This can cause chronic, or congestive heart failure, and/or cardiomyopathy.
CAD is treated with medicines such as statins and beta blockers, and stents are inserted to deal with/remove the blockages. If that fails and the coronary arteries become diseased or blocked again (and again), then the next option is usually bypass.
Mitral valve regurgitation (MVR) is leakage past the mitral valve and if the leakage is bad enough, the condition can lead to heart disease or failure. Usually the mitral valve is repaired or replaced before heart failure occurs.
Pain in the chest that radiates down the arm can be an ominous sign if it is due to coronary artery blockage, and that is the main reason for this pain called angina. Another cause of angina is coronary artery spasms.
Good luck to you and your friend