If a standard user navigates to a HTTPS website with an untrusted certificate in IE, chooses to proceed, then opens the certificate information dialog, the button to Install Certificate is hidden.
If the standard user then starts IE elevated as an administrator user, navigates to the same site and open the certificate dialog the Install Certificate button is available. The default options in the Install Certificate wizard will then trust the certificate only for the administrator user, not the entire system, and the standard user's IE instance still won't trust the website.
However, a standard user doesn't require special permissions to trust a certificate for themselves, the limitation is only imposed by the hidden button.
The Install Certificate button should always be available and should only require elevation when the user chooses the option to trust the certificate for the entire system.