What documents are needed to prove income and company ownership for a visa application when working for a client with a different company name?

One of the requisites for the visa is info on your portfolio. If your client has a website that you do work on but his company name is a different name completely but has no website for that company, and your income, ( although no income requirements are stated its often asked for ), is paid from his company name.

How are we supposed to show proof. Do we need all his company details and registration papers. ? , and how do we show proof that his company owns the website if none of it shows his company name. Just trying to cross i,s and dot the T,s.