Secure sites work just fine in Mac browsers.
Just type in the address and you should be good to go.
What site are you having trouble with? It is probably a totally unrelated issue to the site's security.
Well if Firefox works for him, then you can diagnose the problem much better. In all likelyhood, if Firefox works, but Safari doesn't it means that the security certificates shipped with OS X (they are stored in the keychain and used by Safari) are corrupt or missing. He would need to reinstall those and https: sites should work again.
Firefox comes with its own security certificates, so it wouldn't be affected.