this will always be your awnser if you ask for a standup opinion at an mma forum
Well if you're going for self-defense or MMA, I think boxing, muaythai, kickboxing are the three go to striking arts.
These styles have regular full contact sparring with strikes to the face. Some styles like Kyokushin Karate have hard sparring but they don't allow punches to the face, which can make a huge difference. I seen guys who have participated in tough sports like wrestling or football get hit in the face the first time in boxing turn into little girls. I thought boxing couldn't be possibly worse than judo in terms of pain, but I became a little bitch my first sparring session simply because I wasn't used to getting punched in the face.
I'm not saying other styles like karate and TKD are bad. If someone wants to learn karate for the sake of competing in karate, cool. Karate training will probably add another dimension to your game as well. But if you're a beginner whose goal is MMA, then you need to learn a style that lets you get used to getting hit in the face.
Plus, it'll probably be easier to find serious muay thai or boxing schools. Most karate and TKD schools are mcdojos that serve as a day care center. The chances are you won't find a good san shou or kyokushin school unless you live in a big city.
Ultimately, it comes down to personal preference though. If the TS is serious about learning striking, he should do his own research, check out different schools and see what he likes.