Do you think mentioning in an interview that you know an existing employee is going to count in your favor? Not if that employee has a bad rep.
I once interviewed this guy for a spot on my team. Everything was going along swimmingly until he mentioned that he was friends with an existing employee. Apparently, the employee felt so secure in his internal reputation that he was telling his friends to mention his name when they interviewed.
The problem with that scenario is that the employee did not have a good reputation in the company. He constantly missed deadlines and exercised behaviors that often put people off.
Now, it does not always follow that we are the company we keep. If I am friends with someone, it doesn't mean I share that person's work ethic. To automatically assume so would be illogical. But you know what? Interviewers often assume such things. Right or wrong, they may jump to a conclusion just because something triggers an emotional response in them. In this case, I automatically and subconsciously associated the negative feelings I had for that employee onto the job candidate. I'm not saying that's fair, but that's what happened.
During the interview process, the job candidate was able to present himself in a favorable light. He had a proven track record of work performance that differentiated him from his friend. We ended up hiring the guy, and it was a great hire.
But every time the right thing happens, there are probably a hundred times when it doesn't. People really believe that old adage "birds of a feather flock together." Just be aware that the name you drop could do you more harm than good.