It would be kind of crazy if Ga Min's trainer was Pi Han Ul's father. Would a successful man teach a random kid who wanted to study like it was an obsession? He does Jeet Kune Do which means he's a master in martial arts. I think its plausible too however that would be very sad loss.
There are some routes I think are possible.
One is that he is Pi Han Ul's teacher as well and taught him. That would explain Pi Han Ul's strength and great martial arts.
Two is that he is Pi Han Ul's maternal uncle who raised him and part of the YB gang. Maybe someone in a high position and he's turn to the dark side.
Third is if he was just Ga Min's teacher and ended up aiding in the arrest of the YB gang.
Fourth might be if he indeed is Pi Han Ul's father which would mean that he has no idea that Pi Han Ul is doing evil deeds because he is sick and, in the hospital, or held hostage which is very unlikely because it sounds like PHU is a cruel character. PHU wants to stay under the radar because he doesn't want his father finding out.
