Server ddos

So basically a guy gets mad for losing and starts to ddos a server from our side so that all of my teammates can not move and we lose the rounds. Than we all get kicked for afk and when we reconnect the guy has 35+ kills and they got 9 rounds. How does he even do that and can he get any legal actions from Valve for hacking into their system? Oh and he was the only one who was able to move, even all of his teammates got kicked.