since it might be a suspicious activity
That's where we disagree then. Changing IPs and even changing countries should not be seen as suspicious activity, as far as I am concerned. As I said above, if you use Tor your exit node can and will jump all over the world. Exchanges should not be punishing users for taking their privacy seriously.
If, as in your example, I logged in from Japan, made a deposit in Japanese Yen, traded it to bitcoin, and then the next day logged in from the UK, sold that bitcoin for Great British Pounds, and tried to withdraw it, then sure, that's an argument for suspecting money laundering and completing KYC. A simple IP change is not.
Since, he is trading on a centralized exchange that is expected to happen, maybe they are too strict sad to say but that is the reality now, If he wants more privacy he should choose binance DEX or any decentralized Exchange that any KYC or IPs is not an issue. I guess they are just following this quote "prevention is better than cure" I also believe that our personal data are very precious so I understand both parties.