Hi! First of all, thanks for letting me brush up on my cardiomyopathies
So yes! Someone with a genetically inherited cardiomyopathy can still be alive at age 31! Actually, I just found a paper where the average age of diagnosis for the patients in their study was actually 31.8! So, it’s very likely that your character is still alive and pretty healthy. They might have just started having symptoms of the disease.
As for your congenital heart defects question… so for dilated cardiomyopathy, if you start getting signs of heart failure, you treat with heart failure medications (ACE-Is like lisinopril, beta blockers like atenolol, and diuretics like furosemide). It can also cause arrhythmias of the heart (where your heart doesn’t beat in a normal ba-dup ba-dup rhythm) and you need to take medications for that as well. While some people may opt to get a heart transplant to cure it, that’s a big decision. So one scenario I could see is someone using medications to treat this illness while maybe waiting for/deciding to get a heart transplant, and then get sick of taking all their meds, stop taking an antiarrythmia medication, and then essentially their heart goes into an abnormal rhythm, they require CPR, etc etc… and they could die.
There’s another condition called Hypertrophic Cardiomyopathy. This is also inherited. Unlike in dilated cardiomyopathies (where the heart wall is floppy), in this one, the heart muscle is too thick. You actually can hear a murmur when listening with a stethoscope. If someone has this disease and has symptoms, they also should take medications. Not taking them can cause the disease to progress, etc. However, the interesting thing about HCM is that patients can also suddenly die if they do extreme exercise. They actually tell these patients not to play sports! In Italy (where soccer is a huge sport), they actually screen potential soccer players for this, because they don’t want players to suddenly die while out on the field!
Finally, you can inherit arrhythmia syndromes where you don’t have anything physically wrong with the shape of your heart, but on a molecular level, things don’t work perfectly, so you’re at risk for getting an arrythmia and suddenly dying. An example for this is called “long QT syndrome.” All of these patients should take a beta blocker, as well as watch out for certain medications that can “lengthen their QT” and essentially cause an arrhythmia. If they don’t take their medication, they are at a higher risk of getting a fatal aryhthmia.
Soo… this was a long, rambling post. In general, I think your best bet for killing off someone who was born with a heart condition who didn’t take their medication is to give someone an aryhthmia disorder, since arythmias strike fast and kill quickly. Structural issues with the heart (like cardiomyopathies) can also cause arrythmias, as can things like long QT syndrome.