In Webmasters/Search Console, claim both versions of your client’s website (www.url.com and url.com) and verify both. Then go to your client’s preferred version of their website; usually it will be the ‘www’ version. Click the gear in the right-hand corner and choose Site Settings. Go to the section where it states ‘Display URLs As’ and choose your preferred version. You only need to do this for your preferred version. These settings will carry over to the other version that was not chosen. This is so Google can index your website and know that both versions are from your company.