I agree with other answers that you should not do the review. Here is an example of conflict of interest: http://www.sigmod2015.org/calls_papers_sigmod_research.shtml
A paper author has a conflict of interest with a PC member when and
only when one or more of the following conditions holds:
The PC member is a co-author of the paper.
The PC member has been a co-worker in the same company or university within the past two years
The PC member has been a collaborator within the past two years.
The PC member is or was the author's primary thesis advisor, no matter how long ago.
The author is or was the PC member's primary thesis advisor, no matter how long ago.
The PC member is a relative or close personal friend of the author.
Papers with incorrect or incomplete conflict of interest information
as of the submission closing time are subject to immediate rejection.
Another concern is that your supervisor can recognize your writing style in the review, and this can lead to awkward situation. This is very likely since you and him/her probably co-authored a paper before, you send him/her emails everyday. You may have a habit of language usage that you yourself don't recognize, but everyone else do.
Sometime, I do review for conferences that adopt double-blinded review, and I can correctly guess the authors. Because I read a lot of their papers in the past, and unintentionally train a classifier in my mind to recognize their writing style :).