Difficult Conversation With manipulative Employee

I have an employee that I have to have a difficult conversation with regarding their attitude towards their subordinates. This individual is manipulative and supposedly oblivious to the way they have been treating others. I have spoken with this person twice in the past six months and both times have come from a place of friendship. These conversations have not resolved the issue and it has escalated to the point that if this issue continues it will quickly be beyond my reach. I have one more opportunity to get through, but do not want to be emotionally manipulated out of expressing the ‘truth’ of the situation to this individual. Is there any advice you can share in how to avoid being dragged down into the sphere of their manipulation?