Vpn, change mac address in router. Super easy to do add my skype romulusares I can most likely show you.
VPNs don't work if they've already got your IP... Changing MAC address has no effect on some (mainstream companies) routers. My recommendation is to tell your friend there to call his ISP and find the logs of DDoS. They may be able to file a report on the DDoSer to his ISP. However, that's very unlikely that that'll happen. If he's using a paid booter to DoS/DDoS they'll never get the true IP.
Turn your booter off and on then it resets your public IP that will stop it but before that get a VPN add me on skype if u want one of those DDoSed
If you run your own botnet and they will do all the work and your ip is hidden all the bots ip will be there but no yours and it cant be traced back to you.