Do you Warn your Client about the DDos attacks if he is getting on VPS? What would you suggest him to protect VPS from ddos attacks?
Suggest your client to use cloudflare. They provide advanced DDOS protection. Here is a detailed article from cloudflare. https://www.cloudflare.com/ddos
Stay save to safe your website from hackers.Do not give your pass to any unknown person and use security bsystem for your individual website.
It's always a good idea to keep your customers posted on issues that you notice. Keep in mind, that a responsible providers will have procedures in place to prevent or mitigate these attacks, otherwise an attack on one customer could jepordise the entire network. DDoS Protection at the host-level is becoming a standard, as attacks are becoming more common and frequent.
If your network does not have hardware protection then the best thing you can do is help/tell them how to setup some iptables rules or a firewall like csf to help prevent ddos attacks. You could advise them they could hide their server IP with a service like cloudflare to prevent layer 4 ddos attacks