Patrick is probably right. Cardiomyopathy is serious, often can be only managed. How well that turns out is dependent on age and many other factors. One should of course attempt to align all the favorable lifestyle factors. Nonetheless, trumpet is a physically demanding instrument. While the overall exertion may not be greater than riding a bicycle, there is considerable pressure in the lungs, which in turn, puts pressure on the heart. That kind of pressure on an enlarged, floppy heart, with possibly an already reduced ejection fraction, is not advisable. Its effects are entirely consistent with the symptoms described by the OP. It is very possible that Dan will have to pursue another instrument. I doubt that any cardiologist will tell him it's OK to continue playing trumpet, especially in light of the recent decline.