Unfortunately, there’s almost no crossover between tourney competition and live combat. So, he or she is developing two separate skill sets and that means they’ll have to train twice as hard. Self-defense taekwondo relies mostly on hand strikes, using the legs as a base. All the kicking done is low-line, to the shin, knee, ankle, and groin instead of to the stomach, chest, and head. The reason for this is because kicks rely on friction to function and when faced with a variety of terrain, it’s very easy for the kicker to fall over. Once you end up on the ground in a fight and you’re opponent is still standing, you’re done (and not in the nice, everybody stop fighting way), so it’s better not to take risks.
It might sound funny to say it, but being good at tournament sparring will most likely hurt your character’s ability to do general fighting (or vice versa). The reason for this is that because whichever they do the most of, their minds will settle on that variation (in this case, it’s most likely tournament) and they’ll roll with the kind of combat that they’re familiar with. Depending on their opponent, this can leave them vulnerable to people who don’t play by the rules that they’re used to. It works much like the historical Norman knights versus their Saxon foot soldiers. While it was possible for a knight to lose his life in combat (and many did), if they were captured they could expect to be ransomed back to their family or liege for a purse. Their version of combat had a complex set of rules which they naturally expected to apply to them, which made combat a little less life or death and a little more game. A foot soldier had no such luxury, if they were caught, there was a likelihood they would be hanged, have their eyes put out, their tongues cut out, everything and anything that a Norman noble would not inflict upon another.
You can apply this back to sport fighters versus those who have actually been trained for combat. The mental expectations that they’ll have when going into combat are going to be different from the expectation their opponent has and those expectations of rules (even in situations where those don’t apply because it’s what they are familiar with), beyond their general skill, are what can hurt a martial artist the most when fighting for their life.
One can achieve their first degree of black belt in only three years, but in Taekwondo, the black belt itself is not a symbol of mastery. The black belt has ten ranks and requires a lifetime of study. 2nd is five or more years of training, third is a sign of seven or more, and so on. It’s a mistake to assume that just because your character is a black belt that they know everything there is to know.
WTF (World Taekwon Do) recognized tournaments (the traditional point sparring) all use the same rules, the rules for the Olympics are the same way. Fighters who take their taekwondo on the road to other kinds of fights (MMA or underground street brawls) will obviously be used to different rules. An example would be Cung Lee from StrikeForce and MMA. So your character could be on route to doing different things with their fighting if they lose interest in traditional tournaments.
I hope that helps!