Many companies have a different line on whether or not it's appropriate to be having friends at work. Some have strict non-fraternization rules while others strongly encourage it as part of their work culture. Officevibe made an infographic called '11 Incredible Reasons Why Having Friends at Work is Important,' clearly taking the side it is more than ok.
Based out of Montreal, the startup lists a number of stats, like 50% of employees with a work best friend felt a stronger connection to the company. In fact, over half the statistics mentioned are concerned with having a best friend at work. The overall thesis is that office friendships have a direct link with engagement and productivity at work. Simple things like larger lunch tables can increase productivity and morale by over 25%.