Exodus 20:16 tells us that we should not bear false witness against our neighbor.
This means, whether you like someone or not, if they have not done you wrong in any way, and someone asks you for a reference of them, then that is what you should tell them. If they were a little late in paying you for a debt owed, but you think they are an awesome person, you should not let your feelings for them dictate your truthful answer.
Too many times, events are dictated by cliquish behavior. When people voice opinions based upon other people they are involved with, the truth gets clouded and the results can be disastrous.
A good example of this is in the political realm. People will vote for their party line regardless of the actions of the individual they are voting for. When you vote for someone, you are bearing witness of what you know about them. The Bible says to be a truthful witness.