my friend who is a manager had this experience once.

he once had a subordinate with work attitude problem (always push his task to other colleague and likes to borrow money). later on, this problematic subordinate wanted to hop to other company.

the other company contacted my friend about this subordinate's history and he did not say anything bad about him. he just mentioned briefly that this guy is an all rounder. the reason is because my friend would prefer if the other company take him in than to stay in current company.

needless to say, the problematic employee got the offer and resigned a month later.

in other words, contacting a previous employer may not be a good move too if they want him out desperately. just put in a few good words and let the employee move out, hoping he will never come back.