I happen to be a network engineer. If anything below doesn't make any sense, just ignore it, it probably doesn't apply...My 2c is that the message may be coming from the "catch-all" portion of the try-catch error trap. Why would the 6D communicate over SSL in a LAN as the default method of connecting?
Are you connecting to a WPA/WPA2-EAP network?
Does the radius/IAS server have a self-signed or enterprise CA cert without a trusted root?
Or is the cert from a "real" CA?
It wouldn't be the default in most situations, but with either EAP-TLS or PEAP with EAP-TLS, SSL is mandatory. If he's trying to connect to a corporate wireless network that would be a likely configuration. If he's connecting to home wireless it's highly unlikely; or in the event his home wireless is configured to use WPA Enterprise, chances are anyone that would have the knowledge to set it up probably would have realized the problem and subsequent solution before asking questions here.
That all being said... there was so little information given, it's unclear if this error is occurring during wireless authentication, or if that is succeeding and it's getting the error when the EOS utility is trying to connect to the camera (or vice versa).