It is not a good idea in a cover letter to mention another person whom the employer knows.

You can mention them if it is OK that they notified you of the position and you feel confident that they have a good reputation within the company. And I would mention it only briefly by saying something along the lines of I have the pleasure of knowing Mr. Jones, who informed me there is an open position within your organization. He speaks very highly of your company and I would be interested in seeing if there's an opportunity for me.