He was probably flexing and thought that, considering your age you might not know anything about Bitcoin. The most interesting thing would have been if you had stopped him while he was importing the seed phrase and asked him “Are you importing the seed phrase into the wallet?” He would have been shocked right then and you would have pointed out his mistake.
Like there are CCTV cameras everywhere here and many people are watching. If someone sees your seed phrase they can take all your funds. If you had said something like that I think he would remember it for the rest of his life.
Well, you have a point but honestly, I wouldn't have confronted him either.
I respect people's privacy. The fact they are doing something foolish in public doesn't give us the right to invade their privacy. I will do the same thing minding my business. His money, his wallet and of course his decision. If he loses them, he will learn from his mistakes. Experience is the best teacher, people tend to learn when they have first hand experience.
And we can not even tell if it's a set up to bait others. What if he's actually waiting for someone to approach him and then accuse them of public harassment or something? Why were you looking at his phone in the first place? We should always consider the consequences of our actions first before we act.