Use of a clause without coordinating conjunction

The sentence your friend wrote is incorrect because it joins two independent clauses with a comma. In other words, you could replace the comma with a period and get two grammatically correct sentences.

Your version of the sentence, however is also incorrect. I'm assuming that's because you made some typos.

There are a few easy ways to fix the sentence. You could replace the comma with a period (although that might break things up).

As shown above, he misses someone mysterious, whoever that is, which confirms to us that the feeling is not happiness nor depression. It is emptiness.

You could also replace the comma with a semicolon. This is my favorite solution.

As shown above, he misses someone mysterious, whoever that is, which confirms to us that the feeling is not happiness nor depression; it is emptiness.

The simplest thing to do would be to add a conjunction before the second clause.

As shown above, he misses someone mysterious, whoever that is, which confirms to us that the feeling is not happiness nor depression, but it is emptiness.

The above variation could be improved by using a conjunctive adverb instead of a conjunction, but that's really up to your friend.

For more information on the correct use of commas, I recommend looking at Perdue Owl