Dear Coworker A,
I just witnessed the boss confront Coworker B about a minor screw-up, after you ratted him out. I know how much you dislike Coworker BA, as you have made no secret about it, and I am sure that you took great delight in the opportunity to make him look bad. However, this does not paint you in the most favorable light, and in my experience this sort of thing tends to come back and bite you in the ###! Now we are all going to have to watch our backs and cover our asses, rather than cover for each other. Thanks for being a team player!