There are some exchanges that do not provide service for US citizens, this is not because of the exchange but because of US government that do not allow that. US have exchanges of their own, I will do not know if Kucoin support US. Even if Kucoin support US and providing services to US citizens that register with them, I am still certain that they will not be able to use Kucoin future trading.
But this is what I saw online:
The United States has strict regulations for cryptocurrency exchanges, and KuCoin is not licensed to operate in the states.
KuCoin is not licensed in the United States, but Coinbase is licensed in the U.S.; KuCoin offers 600+ cryptocurrencies.
I will advice you to use the exchanges available in US and know that those exchanges may not support future trading.
I'm afraid your only option is to use a VPN and hope that your account doesn't get locked. (Though I'm not from the US, I've been using KuCoin with a VPN for a while now). Proceed with caution though. Pretty difficult to get access to leverage in the US as far as I know.
I will advice him not to use VPN at all, access those exchanges from US is not legal in US.