I am about to take on a client who is a mutual friend of someone I know. He has a legit business, but one of his competitors somehow complained to google and got his whole site blocked. 

He's starting a different venture now, but I worry - what if this happened to another site I am hosting on a vps for example. Would the fact that I have a blocked site on my server affect my other clients? Could their sites be blocked too for using the same IP address? What are the risks involved?