Ask HN: Protection Against Home Router DDoS

My friend is a live streamer on Twitch, and whenever he plays on Nintendo online with viewers, there is one viewer who has been sneaking in, somehow obtaining his IP address (Nintendo online security flaws?) and DDOSing his, shutting down his internet. So far every time it's happened, he has had to call his ISP and request they manually change his IP to end the attack. This is not my area of expertise, but I really want to help him. Does anyone have any advice? Is there a relatively simple way to Geoblock traffic outside a few select countries (USA, CA, UK, etc.)? Any help appreciated!