I have no idea what "underdeveloped" in a adult means... too small? something missing?
The size of the heart varies and not just by size of the individual. I think this is what we see in some athletes, say a marathon runner, who can get enough oxygen to the muscles over a long period of time...they have an unusual cardiovascular system..which may include larger hearts and lungs, I believe. I mention this only as a statement that a "small" physical heart doesn't make one less of a person, but may limit there ability to do extreme athletics.
The Cleveland Clinic is a top heart hospital, and would be an excellent place to contact. I believe they provide a means to contact them over the web, via their web site. They also offer, for a fee, consultation based on medical facts provided by your doctor(s)... a "second opinion" for example.
It looks like your friend's first ablation didn't work very well. The Cleveland Clinic would be an excellent place for him to go. I don't think he would need to see a pediatric cardiologist, just a good electrrophysiologist. That's the only place he's going to get the answers he needs. I wish him good luck and good care.