Symantec Endpoint Protection 11.0 RU 5 {a.k.a. 11.0.5} is required for compatibility with Windows 7
To download the full version, which is required to install the software for the first time on a Windows 7 machine, you need the serial number that is tied to the customer's licenses. It is typically printed on their certificate.
However, in this case, you mentioned the customer does not have their certificate.
In that case I would recommend you have the customer call Customer Care at 1-800-721-3934
They can search for the customer's licenses and determine if their support contract is current.