OK, I have to chime in here. It is Social Engineering through and through. Let's break down the question from your friend (which forgive me for saying doesn't look like a verbatim quote
A person enters an organization pretending to be an electrician and gets access to the Data Center and completes his work . In the whole procedure he does not talk to a single person .
First of all, your use of the word "pretending" states everything one needs to know. The person was not a real electrician but was using it to fool humans.
Second is the issue of whether it is physical or not. Based on your quote, we don't have enough info. It only states the he, "gets access to the Data Center." He could have picked locks, tailgated, broken down the door, utilized his uniform or any number of other methods, but we simply don't know unless there is more you are leaving out.
Now onto your response:
Social Engg. means that you interact with a human being , and in the case that person went straight to the data center
Just because he did not talk to someone doesn't mean he didn't interact. I'm sure someone saw him as the point seems to be that he didn't have to talk becasue of the uniform. That is interaction. Someone saw him, assumed he was ok, access was attained. Eye contact, body languagem facial expressions... all forms on interaction without using words at all.
If this is someone who is not yet a CISSP and is prepping for the exam, the answer will be SE all the way. If truly already a CISSP (as you state below), I'd be interested in the reason for the question and what expertise your friend may have to make the CISSP go to him. Just curious.