A co-hater is a co-worker who hates on their colleagues. Usually the co-hater is jealous of a job well done. They are often passed over for promotion and resent those who do get promotions or other accolades. In some extreme instances, the co-hater won't acknowledge their colleagues in casual conversation.
I told Lisa about my promotion and she went co-hater on me and ignored me for the rest of the day!