How serious is marijuana for people with dilated cardio myopathy?

Very. Any inhaled pollutant is not healthy for a cardiomyopathy patient. Using weed is not healthy anyway. Best to discontinue any form of smoking for your brain and heart's sake.
Seriously bad idea. Among the bad things smoking marijuana does to the heart and lungs, the quick rise in heart rate and blood pressure, with increased demand for oxygen by the contracting heart muscle, followed by vasodilation (blood vessels relax and dilate) with a drop in blood pressure, can add a lot of stress onto a heart already weak from cardiomyopathy.