Review too short and vague to be useful My friend submitted a paper to a well-known computer science conference. One of the reviews was very short - only 2.5 lines of text, extremely general and vague, and rejected the paper. The review is so meaningless, that the author cannot even respond to it, or use it to improve the paper. Additionally, the review includes little justification to the rejection recommendation.

The other two reviews were balanced - there was one positive and one negative review.

The author feels that the short review should get a smaller weight in the final decision. How can he communicate this to the conference organizers? Is it a good idea to contact them directly by email?

The conference is organized by an automatic web-based submission system that allows the author to write a single response to the reviewers, but it is not clear if this response gets to the organizers or only to the reviewers.
Unfortunately, when submitting to a conference, you don't have control over the process to the detail you are talking about. It will probably be the case that the useless review does indeed get less weight during discussions (unless the reviewer happens to be one of those reviewers who holds a lot of sway and can get a paper rejected with a vague and general review!).

I would urge against sending an email to the conference chair / organizers -- unless you are extremely tactful, any attempt to sway them with this method will come off as whiny and will only hurt your case (or, rather, your friend's case).

The best option is to write a rebuttal as you normally would: fix all the critical comments from the other reviewers, address each comment specifically in your rebuttal response, and thank the reviewers for their consideration. The good news is that you got one positive review.  While this is not normally sufficient for a paper acceptance at a very competitive CS conference, you have the chance to improve the paper as much as you can, and send in your rebuttal, and you should make the most of that opportunity.